intel: Return failure properly in the texsubimage blit path.
intel: Return failure properly in the texsubimage blit path.

We assert that failure doesn't happen, but it fixes a warning in the
release build and it would at least give working behavior for a user by
falling back to the normal texsubimage path.

Reviewed-by: Matt Turner <789b315743a28dd066a4ba1459c35951c291d8a6@gmail.com>

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