mesa/samplerobj: Avoid crash in sampler query if texture unit is disabled

mesa/samplerobj: Avoid crash in sampler query if texture unit is disabled

Sampler queries are so far made only for enabled texture unit. But if
any code would query sampler before checking texture unit state that
would result to NULL deference.

Making the inline helper easier to use with NULL check makes a lot sense
because compiler is likely to combine the checks for the current texture.

Signed-off-by: Pauli Nieminen <pauli.nieminen@linux.intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>