main/texobj: Check that texture id > 0 before looking it up in hash-table

main/texobj: Check that texture id > 0 before looking it up in hash-table

_mesa_lookup_texture_err() is not currently checking that the
texture-id can be zero, but _mesa_HashLookup() doesn't expect the key
to be zero, and will fail an assertion.

Considering that _mesa_lookup_texture_err() is called from
_mesa_GetTextureImage and _mesa_GetTextureSubImage with user provided
arguments, we must validate the texture-id before looking it up in the
hash-table.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>