core: Fix gpgrt_wait_processes, by skipping invalid PID.
* src/spawn-posix.c (_gpgrt_wait_processes): Skip invalid PID.
The API itself is not good to handle multiple processes. Given the
API, skipping invalid PID is better for usefulness.
- GnuPG-bug-id: T5381
- Reported-by: Jakub Jelen <jjelen@redhat.com>
- Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>