glsl: fix UNIFORM_BUFFER_START or UNIFORM_BUFFER_SIZE query when no buffer object...

glsl: fix UNIFORM_BUFFER_START or UNIFORM_BUFFER_SIZE query when no buffer object is bound

According to ARB_uniform_buffer_object spec:

"If the parameter (starting offset or size) was not specified when the
 buffer object was bound (e.g. if bound with BindBufferBase), or if no
 buffer object is bound to <index>, zero is returned."

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>