[MERGE #5957 @MikeHolman] fix ObjectBeforeCollectCallback unittests
[MERGE #5957 @MikeHolman] fix ObjectBeforeCollectCallback unittests

Merge pull request #5957 from MikeHolman:objbeforecollect

nullptr should clear out existing callbacks
explicitly make oom when merging callback lists unrecoverable (we were asserting that OOM was unhandled)

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