Fix prvWriteMessageToBuffer on big endian (#391)

Fix prvWriteMessageToBuffer on big endian (#391)

prvWriteMessageToBuffer wrote the first sbBYTES_TO_STORE_MESSAGE_LENGTH
bytes of the size_t-typed length to the buffer as the data length. While
this functions on little endian, it copies the wrong bytes on big
endian. This fix converts the length to configMESSAGE_BUFFER_LENGTH_TYPE
first, and then copies the exact amount, thus fixing the issue.
Additionally it adds an assert to verify the size is not greater than
the max value of configMESSAGE_BUFFER_LENGTH_TYPE; previously this would
truncate silently.

Co-authored-by: Gaurav-Aggarwal-AWS <33462878+aggarg@users.noreply.github.com>