ac: do not crash when the buffer data format is invalid

ac: do not crash when the buffer data format is invalid

This might happen when a pipeline doesn't define the vertex input
state, so the buffer data format is 0 (aka INVALID).

This fixes crashes when compiling some shaders on GFX10.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>