GRMustacheRenderNSFastEnumeration fix crash handling rendering error

I copied the inserted code from other parts of the codebase where similar precautions are necessary (e.g. `GRMustacheAccumulatorTag.m`).

I included a test, but I'm not sure how the tests folder is organized, or if a test is even desired for this issue. Let me know if I can reorganize things for you.