mesa/main: bail earlier for size == 0 in _mesa_clear_buffer_sub_data

mesa/main: bail earlier for size == 0 in _mesa_clear_buffer_sub_data

Note that the conversion of the clear data (when data != NULL) can fail due
to an out of memory condition, but it does not check any error conditions
mandated by the spec. Therefore, it is safe to skip when size == 0.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>