gallium: Make pipe_query_result::batch array length non-zero.

gallium: Make pipe_query_result::batch array length non-zero.

Zero length arrays are non standard:

   warning C4200: nonstandard extension used : zero-sized array in struct/union
   Cannot generate copy-ctor or copy-assignment operator when UDT contains a zero-sized array

And all code does `N * sizeof query_result->batch[0]`, so it should work
exactly the same.

Reviewed-by: Roland Scheidegger <sroland@vmware.com>