run-command.c: fix broken list iteration in clear_child_for_cleanup

Iterate through children_to_clean using 'next' fields but with an
extra level of indirection. This allows us to update the chain when
we remove a child and saves us managing several variables around
the loop mechanism.

Signed-off-by: David Gould <david@optimisefitness.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -49,13 +49,14 @@ static void mark_child_for_cleanup(pid_t pid)
 
 static void clear_child_for_cleanup(pid_t pid)
 {
-	struct child_to_clean **last, *p;
+	struct child_to_clean **pp;
 
-	last = &children_to_clean;
-	for (p = children_to_clean; p; p = p->next) {
-		if (p->pid == pid) {
-			*last = p->next;
-			free(p);
+	for (pp = &children_to_clean; *pp; pp = &(*pp)->next) {
+		struct child_to_clean *clean_me = *pp;
+
+		if (clean_me->pid == pid) {
+			*pp = clean_me->next;
+			free(clean_me);
 			return;
 		}
 	}