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.