Fix possibly uninitilized variable handling.

If malloc errors are ignored (which say aren't by default and never
should be) malloc in the old, slow form might use an uninitialized
variable.

malloc/malloc.c

@@ -4306,6 +4306,7 @@ _int_malloc(mstate av, size_t bytes)
 	  errstr = "malloc(): memory corruption (fast)";
 	errout:
 	  malloc_printerr (check_action, errstr, chunk2mem (victim));
+	  return NULL;
 	}
 #ifndef ATOMIC_FASTBINS
       *fb = victim->fd;