posix: Fix posix_spawn invalid memory access

Current Linux posix_spawn spawn do not test if the pid argument is
valid before trying to update it for success case.  This patch fixes
it.

Tested on x86_64 and i686.

	* sysdeps/unix/sysv/linux/spawni.c (__spawnix): Fix invalid memory
	access where posix_spawn success and pid argument is null.
	* posix/tst-spawn.c (do_test): Add posix_spawn null pid argument for
	success case.

ChangeLog

@@ -1,3 +1,10 @@
+2016-03-20  Adhemerval Zanella  <adhemerval.zanella@linaro.org>
+
+	* sysdeps/unix/sysv/linux/spawni.c (__spawnix): Fix invalid memory
+	access where posix_spawn success and pid argument is null.
+	* posix/tst-spawn.c (do_test): Add posix_spawn null pid argument for
+	success case.
+
 2016-03-20  Samuel Thibault  <samuel.thibault@ens-lyon.org>:
 
 	* sysdeps/mach/hurd/i386/c++-types.data: New file.

posix/tst-spawn.c

@@ -257,6 +257,10 @@ do_test (int argc, char *argv[])
    if (posix_spawn (&pid, argv[1], &actions, NULL, spargv, environ) != 0)
      error (EXIT_FAILURE, errno, "posix_spawn");
 
+   /* Same test but with a NULL pid argument.  */
+   if (posix_spawn (NULL, argv[1], &actions, NULL, spargv, environ) != 0)
+     error (EXIT_FAILURE, errno, "posix_spawn");
+
    /* Cleanup.  */
    if (posix_spawn_file_actions_destroy (&actions) != 0)
      error (EXIT_FAILURE, errno, "posix_spawn_file_actions_destroy");

sysdeps/unix/sysv/linux/spawni.c

@@ -381,7 +381,7 @@ __spawnix (pid_t * pid, const char *file,
 
   close_not_cancel (args.pipe[0]);
 
-  if (!ec && new_pid)
+  if (!ec && pid)
     *pid = new_pid;
 
   __sigprocmask (SIG_SETMASK, &args.oldmask, 0);