transactionrecord.cpp: fix false mint-by-burn bug

Updated TransactionRecord::decomposeTransaction to hopefully correct the bug where transactions were incorrectly displaying as "burn-by-mint" even though they weren't. It was not checking whether the transaction was the coinbase transaction, so causing all transactions in PoB blocks to mark as "mint-by-burn".
Merge pull request #4 from tbartlett0/master

transactionrecord.cpp: fix false mint-by-burn bug