Don't leak olinep if malloc() fails.

If malloc() fails to allocate linep, then free olinep (if it exists)
before returning to avoid a memory leak.

Reported by:	Coverity
CID:		1016716
Reviewed by:	kib
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D6755

lib/libc/gen/getnetgrent.c

@@ -614,6 +614,8 @@ read_for_group(const char *group)
 					if (linep == NULL) {
 						free(lp->l_groupname);
 						free(lp);
+						if (olen > 0)
+							free(olinep);
 						return (NULL);
 					}
 					if (olen > 0) {