mesa: add missing error checks in _mesa_GetObject[Ptr]Label()

mesa: add missing error checks in _mesa_GetObject[Ptr]Label()

Error checking bufSize isn't mentioned in the spec, but it is in the
man pages.  However, I believe the man page is incorrect.  Typically,
GL functions that take GLsizei parameters check that they're positive
or non-negative.  Negative values don't make sense here.

A spec bug has been filed with Khronos/ARB.

v2: check for negative values, not <= 0.