[PR95456] avoid memcpy (_, NULL, 0) in gcc.c

Some newly-added code in gcc.c might call memcpy with a NULL source
pointer and zero-length inputs.  Avoid such calls by rearranging the
code a little.


for  gcc/ChangeLog

	PR driver/95456
	* gcc.c (do_spec_1): Don't call memcpy (_, NULL, 0).