[1.8>master] [MERGE #4402 @Penguinwizzard] MSFT:14963804:Use correct sizes for BufferBuilder failfasts
[1.8>master] [MERGE #4402 @Penguinwizzard] MSFT:14963804:Use correct sizes for BufferBuilder failfasts

Merge pull request #4402 from Penguinwizzard:prefast_bufferbuilder

We weren't hitting this case before (since there's only one place
that uses BufferBuilder, and it allocates space correctly), but a
future use should avoid unsafe usage of this code now.

Microsoft/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,Microsoft/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore,mrkmarron/ChakraCore