PowerPC md_end: Don't htab_delete(NULL)

It might be possible to hit md_end before md_begin is called, don't
segfault if so.  Also, remove a useless check.

	* gas/config/tc-ppc.c (insn_calloc): Remove needless overflow
	check.
	(ppc_md_end): Check ppc_hash before deleting.  Clear ppc_hash.