radv: don't assert on empty hash table

radv: don't assert on empty hash table

Currently if table_size is 0, it's falling through to:

unreachable("hash table should never be full");

But table_size can be 0 when RADV_DEBUG=nocache is set, or when the
table allocation fails (which is not considered an error).

Fixes: f4e499ec791 "radv: add initial non-conformant radv vulkan driver"
Signed-off-by: Grazvydas Ignotas <notasas@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>