spirv: Fix loading an entire block at once.

There is no chain, so  checking the length ends with a SEGFAULT.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103579


Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit b926da24)
[Andres Gomez: 16 bits types and vtn_assert/fail not yet in 17.2]
Signed-off-by: Andres Gomez <agomez@igalia.com>

Conflicts:
	src/compiler/spirv/vtn_variables.c