anv: do not try to ref/unref NULL shaders

anv: do not try to ref/unref NULL shaders

This situation can happen if we failed to allocate memory for the shader.

v2:
 - We shouldn't see NULL shaders in anv_shader_bin_ref so we should not check
   for that (Jason). Make sure that callers don't attempt to call this
   function with a NULL shader and assert that this never happens (Iago).

v3:
 - All callers to anv_shader_bin_unref seem to check for NULL before calling,
   so just assert that it is not NULL (Topi)

Reviewed-by: Topi Pohjolainen <topi.pohjolainen@intel.com>