Fixed a segfault when waitpid returned 0, should fix the url crash

Fixed the segfault that occured here: https://github.com/p-e-w/finalterm/issues/215, though it might actually be an issue about mixing the glib api for processes and manually handling them.

When launch_default_for_uri is called, it sends SIGCHLD, which triggers the signal handler on a process that it cannot find.