posix: Fix segfault in maybe_script_execute

This patch is essentially 28669f86 adjusted for the generic
implementation.

Checked on x86_64-linux-gnu with Linux spawni.c removed.  The only
failure is posix/tst-spawn3, which is expected.

	[BZ #23913]
	* sysdeps/posix/spawni.c (maybe_script_execute):
	Increment size of new_argv by one.