Added comments referencing multiple CVEs in tests and production code.

This commit adds comments referencing multiple CVEs both in production and test code.
CVEs covered in this commit:

CVE-2010-5137
CVE-2010-5139
CVE-2010-5141
CVE-2012-1909
CVE-2012-2459
CVE-2012-3789
CVE-2018-17144

test/functional/mempool_accept.py

@@ -212,6 +212,7 @@ class MempoolAcceptanceTest(BitcoinTestFramework):
             rawtxs=[tx.serialize().hex()],
         )
 
+        # The following two validations prevent overflow of the output amounts (see CVE-2010-5139).
         self.log.info('A transaction with too large output value')
         tx.deserialize(BytesIO(hex_str_to_bytes(raw_tx_reference)))
         tx.vout[0].nValue = 21000000 * COIN + 1