Fix a NULL pointer passed to memcpy
Fix a NULL pointer passed to memcpy

Since we copy 0 bytes, this is benign, however the sanitizer barks
I used CC=clang ./mvm with added CFLAGS -fsanitize=undefined
> opensmalltalk/platforms/unix/vm-display-X11/sqUnixX11.c:1267:24: runtime error:
> null pointer passed as argument 2, which is declared to never be null
> /usr/include/string.h:43:28: note: nonnull attribute specified here

timfel/squeakvm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm,OpenSmalltalk/vm,timfel/squeakvm,timfel/squeakvm,timfel/squeakvm,OpenSmalltalk/vm