mesa: Adds missing error condition in _mesa_check_sample_count()

mesa: Adds missing error condition in _mesa_check_sample_count()

This corrects a trivial error introduced in commit
19252fee46b835cb4f6b1cce18d7737d62b64a2e. That patch was merged recently
and omits one condition (that 'samples' is greater than zero) in one of
the error checks. That error will definitely cause regressions.

Also corrects the reference to the specification above the error check,
which was wrongly quoting OpenGL instead of OpenGL-ES.

Reviewed-by: Martin Peres <martin.peres@linux.intel.com>