Steps to reproduce:
1. Start dia
2. Select the "FS" sheet from the set of possible Dia buttons
3. Select the "Flow =>" button (the leftmost of the three options)
4. Add a new flow arrow to the diagram (of any shape)
5. Press the "Delete" key, or click "Edit -> Delete"


Stack trace:
*** glibc detected *** dia: free(): invalid pointer: 0x0000000002662770 ***
======= Backtrace: =========
/lib/libc.so.6[0x2b93c1012b33]
/lib/libc.so.6(cfree+0x8c)[0x2b93c101627c]
dia(diagram_unselect_object+0x2f)[0x41d88f]
dia(diagram_unselect_objects+0x5c)[0x41d94c]
dia[0x4246b1]
dia(edit_delete_callback+0x3c)[0x42aeec]
/usr/lib/libgobject-2.0.so.0(g_closure_invoke+0x10a)[0x2b93bf9937da]
/usr/lib/libgobject-2.0.so.0[0x2b93bf9a3408]
/usr/lib/libgobject-2.0.so.0(g_signal_emit_valist+0x833)[0x2b93bf9a4843]
/usr/lib/libgobject-2.0.so.0(g_signal_emit+0x83)[0x2b93bf9a4a13]
/usr/lib/libgtk-x11-2.0.so.0(_gtk_action_emit_activate+0x43)[0x2b93bce18f13]
/usr/lib/libgtk-x11-2.0.so.0[0x2b93bce1acd9]
/usr/lib/libgobject-2.0.so.0(g_closure_invoke+0x10a)[0x2b93bf9937da]
/usr/lib/libgobject-2.0.so.0[0x2b93bf9a3408]
/usr/lib/libgobject-2.0.so.0(g_signal_emit_valist+0x607)[0x2b93bf9a4617]
/usr/lib/libgobject-2.0.so.0(g_signal_emit+0x83)[0x2b93bf9a4a13]
/usr/lib/libgtk-x11-2.0.so.0(gtk_accel_group_activate+0xfe)[0x2b93bce1565e]
/usr/lib/libgtk-x11-2.0.so.0(gtk_accel_groups_activate+0xef)[0x2b93bce1575f]
/usr/lib/libgtk-x11-2.0.so.0[0x2b93bcff4f39]
/usr/lib/libgtk-x11-2.0.so.0(_gtk_marshal_BOOLEAN__BOXED+0x5d)[0x2b93bcee668d]
/usr/lib/libgobject-2.0.so.0(g_closure_invoke+0x10a)[0x2b93bf9937da]
/usr/lib/libgobject-2.0.so.0[0x2b93bf9a3a18]
/usr/lib/libgobject-2.0.so.0(g_signal_emit_valist+0x607)[0x2b93bf9a4617]
/usr/lib/libgobject-2.0.so.0(g_signal_emit+0x83)[0x2b93bf9a4a13]
/usr/lib/libgtk-x11-2.0.so.0[0x2b93bcfe413e]
/usr/lib/libgtk-x11-2.0.so.0(gtk_propagate_event+0x1c5)[0x2b93bcedfd45]
/usr/lib/libgtk-x11-2.0.so.0(gtk_main_do_event+0x321)[0x2b93bcee0c91]
/usr/lib/libgdk-x11-2.0.so.0[0x2b93bd38745c]
/usr/lib/libglib-2.0.so.0(g_main_context_dispatch+0x1b4)[0x2b93c0000a14]
/usr/lib/libglib-2.0.so.0[0x2b93c000385d]
/usr/lib/libglib-2.0.so.0(g_main_loop_run+0x1ca)[0x2b93c0003b6a]
/usr/lib/libgtk-x11-2.0.so.0(gtk_main+0xa3)[0x2b93bcee1023]
dia(main+0x25)[0x458795]
/lib/libc.so.6(__libc_start_main+0xf4)[0x2b93c0fc08e4]
dia[0x41d119]
======= Memory map: ========
00400000-00476000 r-xp 00000000 08:01 1806446                            /usr/bin/dia-normal
00676000-0067c000 rw-p 00076000 08:01 1806446                            /usr/bin/dia-normal
0067c000-02756000 rw-p 0067c000 00:00 0                                  [heap]
2b93bc557000-2b93bc573000 r-xp 00000000 08:01 6111234                    /lib/ld-2.5.so
2b93bc573000-2b93bc576000 rw-p 2b93bc573000 00:00 0 
2b93bc772000-2b93bc774000 rw-p 0001b000 08:01 6111234                    /lib/ld-2.5.so
2b93bc774000-2b93bc7fd000 r-xp 00000000 08:01 3870233                    /usr/lib/dia/libdia.so
2b93bc7fd000-2b93bc9fd000 ---p 00089000 08:01 3870233                    /usr/lib/dia/libdia.so
2b93bc9fd000-2b93bca07000 rw-p 00089000 08:01 3870233                    /usr/lib/dia/libdia.so
2b93bca07000-2b93bca0e000 r--s 00000000 08:01 1814206                    /usr/lib/gconv/gconv-modules.cache
2b93bca1e000-2b93bca25000 r-xp 00000000 08:01 6111694                    /lib/libpopt.so.0.0.0
2b93bca25000-2b93bcc25000 ---p 00007000 08:01 6111694                    /lib/libpopt.so.0.0.0
2b93bcc25000-2b93bcc26000 rw-p 00007000 08:01 6111694                    /lib/libpopt.so.0.0.0
2b93bcc26000-2b93bcc9a000 r-xp 00000000 08:01 1806054                    /usr/lib/libfreetype.so.6.3.10
2b93bcc9a000-2b93bcd9a000 ---p 00074000 08:01 1806054                    /usr/lib/libfreetype.so.6.3.10
2b93bcd9a000-2b93bcd9f000 rw-p 00074000 08:01 1806054                    /usr/lib/libfreetype.so.6.3.10
2b93bcd9f000-2b93bd12e000 r-xp 00000000 08:01 1806432                    /usr/lib/libgtk-x11-2.0.so.0.1000.11
2b93bd12e000-2b93bd32d000 ---p 0038f000 08:01 1806432                    /usr/lib/libgtk-x11-2.0.so.0.1000.11
2b93bd32d000-2b93bd338000 rw-p 0038e000 08:01 1806432                    /usr/lib/libgtk-x11-2.0.so.0.1000.11
2b93bd338000-2b93bd33b000 rw-p 2b93bd338000 00:00 0 
2b93bd33b000-2b93bd3d0000 r-xp 00000000 08:01 1806431                    /usr/lib/libgdk-x11-2.0.so.0.1000.11
2b93bd3d0000-2b93bd5cf000 ---p 00095000 08:01 1806431

Other information:
I can get a proper stacktrace if needed; this repros 100% for me on Linux/AMD64 (current Ubuntu Fiesty Fawn).