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>

src/intel/vulkan/anv_private.h

@@ -1556,14 +1556,14 @@ anv_shader_bin_destroy(struct anv_device *device, struct anv_shader_bin *shader)
 static inline void
 anv_shader_bin_ref(struct anv_shader_bin *shader)
 {
-   assert(shader->ref_cnt >= 1);
+   assert(shader && shader->ref_cnt >= 1);
    __sync_fetch_and_add(&shader->ref_cnt, 1);
 }
 
 static inline void
 anv_shader_bin_unref(struct anv_device *device, struct anv_shader_bin *shader)
 {
-   assert(shader->ref_cnt >= 1);
+   assert(shader && shader->ref_cnt >= 1);
    if (__sync_fetch_and_add(&shader->ref_cnt, -1) == 1)
       anv_shader_bin_destroy(device, shader);
 }