common: Fix gnupg_wait_processes.

* common/exechelp-posix.c (gnupg_wait_processes): Loop for r_exitcodes
even if we already see an error.

Cherry-picked master commit of:
eeb3da6eb717ed6a1a1069a7611eb37503e8672d

The value stored by waitpid for exit code is encoded; It requires
decoded by WEXITSTATUS macro, regardless of an error.

For example, when one of processes is already exited and another is
still running, it resulted wrong value of in r_exitcodes[n].

• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>