Fix GDB crash with upstream GCC due to memcpy(NULL, ...)

Fix GDB crash with upstream GCC due to memcpy(NULL, ...)

Similar to qsort(), the glibc version of memcpy() also declares its
arguments with __attribute__(__nonnull__(...)).  If NULL is passed
anyway, upstream GCC's new pass '-fisolate-erroneous-paths' typically
causes a trap in such cases.  I've encountered this with GDB in
chain_candidate() when trying to execute the break.exp test case.

gdb/
2013-11-13  Andreas Arnez  <arnez@linux.vnet.ibm.com>

	* dwarf2loc.c (chain_candidate): Prevent invoking memcpy with
	NULL.