gui: Check return value of para_exec_cmdline_pid().

external_cmd() calls this function to execute the given command.
However, as para_exec_cmdline_pid() may fail, e.g. because the
underlying call to fork() failed due to hitting the RLIMIT_NPROC
resource limit, we must check the return value. The other two command
dispatchers got this right, but external_cmd() did not.