The address might be zero-size, and therefore nl_addr_get_binary_addr()
returns a pointer to a zero-size array.  We don't want to read past the
end of that array.  Since zero-size addresses really mean an address
of all zeros, just make that happen.

As an additional optimization, if the prefix length is zero, the whole
address is host bits and should be cleared.

==30286== Invalid read of size 4
==30286==    at 0x478090: clear_host_address (nm-linux-platform.c:3786)
==30286==    by 0x4784D4: route_search_cache (nm-linux-platform.c:3883)
==30286==    by 0x4785A1: refresh_route (nm-linux-platform.c:3901)
==30286==    by 0x4787B6: ip4_route_delete (nm-linux-platform.c:3978)
==30286==    by 0x47F674: nm_platform_ip4_route_delete (nm-platform.c:1980)
==30286==    by 0x4B279D: _v4_platform_route_delete_default (nm-default-route-manager.c:1122)
==30286==    by 0x4AEF03: _platform_route_sync_flush (nm-default-route-manager.c:320)
==30286==    by 0x4B043E: _resync_all (nm-default-route-manager.c:574)
==30286==    by 0x4B0CA7: _entry_at_idx_remove (nm-default-route-manager.c:631)
==30286==    by 0x4B1A66: _ipx_update_default_route (nm-default-route-manager.c:806)
==30286==    by 0x4B1A9C: nm_default_route_manager_ip4_update_default_route (nm-default-route-manager.c:813)
==30286==    by 0x45C3BC: _cleanup_generic_post (nm-device.c:7143)
==30286==  Address 0xee33514 is 0 bytes after a block of size 20 alloc'd
==30286==    at 0x4C2C080: calloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==30286==    by 0x6B2B0B1: nl_addr_alloc (in /usr/lib/libnl-3.so.200.20.0)
==30286==    by 0x6B2B0E3: nl_addr_build (in /usr/lib/libnl-3.so.200.20.0)
==30286==    by 0x6B2B181: nl_addr_clone (in /usr/lib/libnl-3.so.200.20.0)
==30286==    by 0x66DB0D7: ??? (in /usr/lib/libnl-route-3.so.200.20.0)
==30286==    by 0x6B33CE6: nl_object_clone (in /usr/lib/libnl-3.so.200.20.0)
==30286==    by 0x6B2D303: nl_cache_add (in /usr/lib/libnl-3.so.200.20.0)
==30286==    by 0x472E55: refresh_object (nm-linux-platform.c:1735)
==30286==    by 0x473137: add_object (nm-linux-platform.c:1795)
==30286==    by 0x478373: ip4_route_add (nm-linux-platform.c:3846)
==30286==    by 0x47F375: nm_platform_ip4_route_add (nm-platform.c:1939)
==30286==    by 0x4AEC06: _platform_route_sync_add (nm-default-route-manager.c:254)