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

Diff for: sapi/fpm/fpm/fpm_children.c

@@ -72,6 +72,7 @@ static void fpm_child_close(struct fpm_child_s *child, int in_event_loop) /* {{{
 		if (child->fd_stdout != -1) {
 			close(child->fd_stdout);
 		}
+		fpm_event_del(&child->ev_stdout);
 	}
 
 	if (child->fd_stderr != -1) {
@@ -81,6 +82,7 @@ static void fpm_child_close(struct fpm_child_s *child, int in_event_loop) /* {{{
 		if (child->fd_stderr != -1) {
 			close(child->fd_stderr);
 		}
+		fpm_event_del(&child->ev_stderr);
 	}
 
 	fpm_child_free(child);