libelf: Return an error instead of asserting on an invalid ar file

Upstream elftoolchain ticket 467
http://sourceforge.net/p/elftoolchain/tickets/467/

Reported by:	Alexander Cherepanov
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation

contrib/elftoolchain/libelf/libelf_ar_util.c

@@ -279,7 +279,8 @@ _libelf_ar_open(Elf *e, int reporterror)
 	 */
 	if (arh.ar_name[0] == '/') {
 
-		assert(sz > 0);
+		if (sz == 0)
+			goto error;
 
 		e->e_flags |= LIBELF_F_AR_VARIANT_SVR4;