mesa: add back glGetnUniform*v() overflow error reporting

mesa: add back glGetnUniform*v() overflow error reporting

The error was removed in:

commit 719909698c67c287a393d2380278e7b7495ae018
Author: Ian Romanick <ian.d.romanick@intel.com>
Date:   Tue Oct 18 16:01:49 2011 -0700

    mesa: Rewrite the way uniforms are tracked and handled

The GL_ARB_robustness spec doesn't say the implementation
should truncate the output, so just return after setting
the required error like it did before the above commit.

Also fixup an old comment and add an assert.

NOTE: This is a candidate for the 8.0 branch.