run-command: fix segfault when cleaning forked async process

Callers of the run-command API may mark a child as
"clean_on_exit"; it gets added to a list and killed when the
main process dies.  Since commit 46df6906f
(execv_dashed_external: wait for child on signal death,
2017-01-06), we respect an extra "wait_after_clean" flag,
which we expect to find in the child_process struct.

When Git is built with NO_PTHREADS, we start "struct
async" processes by forking rather than spawning a thread.
The resulting processes get added to the cleanup list but
they don't have a child_process struct, and the cleanup
function ends up dereferencing NULL.

We should notice this case and assume that the processes do
not need to be waited for (i.e., the same behavior they had
before 46df6906f).

Reported-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

run-command.c

@@ -48,7 +48,7 @@ static void cleanup_children(int sig, int in_signal)
 
 		kill(p->pid, sig);
 
-		if (p->process->wait_after_clean) {
+		if (p->process && p->process->wait_after_clean) {
 			p->next = children_to_wait_for;
 			children_to_wait_for = p;
 		} else {