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

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

Upstream GCC's new pass '-fisolate-erroneous-paths' may introduce
traps at places where GCC has determined undefined behavior, e.g. when
passing a NULL pointer to a function that defines this argument as
__attribute__(__nonnull__(...)).  In particular this applies to
uniquify_strings(), because it invokes qsort() with NULL when the
'strings' vector is empty.  I hit this problem on s390x when trying to
execute "break main" on a C program.

gdb/
2013-11-12  Andreas Arnez  <arnez@linux.vnet.ibm.com>

	* objc-lang.c (uniquify_strings): Prevent invoking qsort with
	NULL.