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

src/validation.cpp

@@ -1865,7 +1865,7 @@ bool CChainState::ConnectBlock(const CBlock& block, CValidationState& state, CBl
     // If such overwrites are allowed, coinbases and transactions depending upon those
     // can be duplicated to remove the ability to spend the first instance -- even after
     // being sent to another address.
-    // See BIP30 and http://r6.ca/blog/20120206T005236Z.html for more information.
+    // See BIP30, CVE-2012-1909, and http://r6.ca/blog/20120206T005236Z.html for more information.
     // This logic is not necessary for memory pool transactions, as AcceptToMemoryPool
     // already refuses previously-known transaction ids entirely.
     // This rule was originally applied to all blocks with a timestamp after March 15, 2012, 0:00 UTC.
@@ -3136,6 +3136,7 @@ bool CheckBlock(const CBlock& block, CValidationState& state, const Consensus::P
             return state.Invalid(ValidationInvalidReason::CONSENSUS, false, REJECT_INVALID, "bad-cb-multiple", "more than one coinbase");
 
     // Check transactions
+    // Must check for duplicate inputs (see CVE-2018-17144)
     for (const auto& tx : block.vtx)
         if (!CheckTransaction(*tx, state, true))
             return state.Invalid(state.GetReason(), false, state.GetRejectCode(), state.GetRejectReason(),