listing: list_emit -- Don't forget to zap listerr_head

In list_emit we walk over listerr_head freeing the list,
but the head pointer remain carrying old value. Need
to clean it up once traverse is done to not access
already freed memory later.

https://bugzilla.nasm.us/show_bug.cgi?id=3392538

Reported-by: russvz@comcast.net
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

asm/listing.c

@@ -128,6 +128,7 @@ static void list_emit(void)
 	fprintf(listfp, "  %s\n", le->str);
         nasm_free(le);
     }
+    listerr_head = NULL;
     listerr_tail = &listerr_head;
 }