lib/spawn.c run_command: don't loop forever if waitpid() is returning ECHILD

If SIGCHILD is being ignored, waitpid() will forever error with ECHILD and
this loop with never end, so don't loop if it errors with ECHILD.

lib/spawn.c

@@ -68,6 +68,8 @@ int run_command (const char *cmd, const char *argv[],
 
 	do {
 		wpid = waitpid (pid, status, 0);
+		if ((pid_t)-1 == wpid && errno == ECHILD)
+			break;
 	} while (   ((pid_t)-1 == wpid && errno == EINTR)
 	         || ((pid_t)-1 != wpid && wpid != pid));