[MERGE #3432 @MSLaguana] Fixing debug assert failure in WriteStringCopy
[MERGE #3432 @MSLaguana] Fixing debug assert failure in WriteStringCopy

Merge pull request #3432 from MSLaguana:fixStringCopyErrorChecking

The WriteStringCopy function was accessing string pointers without
setting up error handling correctly, triggering asserts in debug
builds.

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