GL edge cases.

Fix null pointer reads and writes in glGetUniformIndices(). Fix cases where GL layer might throw a JS exception if a GL call fails. Fix null pointer writes if zero is passed as an out pointer in multiple glGet*() calls. @kripken: Could you give this a brief look?
Hmm, I guess that's ok - although any glGet()s should not typically be in a fast path anyway, and a if() there shouldn't be that heavy. However I suppose I agree, doing these checks are checking for programming errors, so I updated the pull request now to do that.