Steps to reproduce:
1. Just compile this:  

#include <gtk/gtk.h>

int main (int argc, char *argv[])
{
        gtk_init (&argc, &argv);

        return(0);
}

2. run it as:
 % ./gtk_bug -1234567890123456789012345678

3. you should get "Segmentation Fault"

Stack trace:
gdb says :

+ Trace 55428

but valgrind is more verbose:

==29250== Invalid read of size 1
==29250==    at 0x1B9047C4: strcpy (mac_replace_strmem.c:198)
==29250==    by 0x1BD8327C: (within /usr/lib/libglib-2.0.so.0.600.2)
==29250==    by 0x1BD83442: g_option_context_parse (in
/usr/lib/libglib-2.0.so.0.600.2)
==29250==    by 0x1BA3A78D: gtk_parse_args (in /usr/lib/libgtk-x11-2.0.so.0.600.2)
==29250==  Address 0x1C14C02C is 0 bytes after a block of size 28 alloc'd
==29250==    at 0x1B904EDD: malloc (vg_replace_malloc.c:131)
==29250==    by 0x1BD7CA36: g_malloc (in /usr/lib/libglib-2.0.so.0.600.2)
==29250==    by 0x1BD83798: g_option_context_parse (in
/usr/lib/libglib-2.0.so.0.600.2)
==29250==    by 0x1BA3A78D: gtk_parse_args (in /usr/lib/libgtk-x11-2.0.so.0.600.2)


Other information:
I think there's a missing "+1" at malloc line in g_option_context_parse? isn't it?

Have a nice day!