Fix GH-8517: FPM child can be possibly freed before stdio event

We should delete event before freeing child so it cannot be triggered
after the child is freed.

Author: bukka

sapi/fpm/fpm/fpm_children.c

@@ -68,6 +68,7 @@ static void fpm_child_close(struct fpm_child_s *child, int in_event_loop) /* {{{
 	if (child->fd_stdout != -1) {
 		if (in_event_loop) {
 			fpm_event_fire(&child->ev_stdout);
+			fpm_event_del(&child->ev_stdout);
 		}
 		if (child->fd_stdout != -1) {
 			close(child->fd_stdout);
@@ -77,6 +78,7 @@ static void fpm_child_close(struct fpm_child_s *child, int in_event_loop) /* {{{
 	if (child->fd_stderr != -1) {
 		if (in_event_loop) {
 			fpm_event_fire(&child->ev_stderr);
+			fpm_event_del(&child->ev_stdout);
 		}
 		if (child->fd_stderr != -1) {
 			close(child->fd_stderr);