Eliminate unnecessary call to CheckBlock (#1498)

ProcessNewBlock would return failure early if CheckBlock failed, before
calling AcceptBlock.  AcceptBlock also calls CheckBlock, and upon failure
would update mapBlockIndex to indicate that a block was failed.  By returning
early in ProcessNewBlock, we were not marking blocks that fail a check in
CheckBlock as permanently failed, and thus would continue to re-request and
reprocess them.

src/main.cpp

@@ -4013,16 +4013,10 @@ static bool IsSuperMajority(int minVersion, const CBlockIndex* pstart, unsigned
 
 bool ProcessNewBlock(CValidationState& state, const CChainParams& chainparams, const CNode* pfrom, const CBlock* pblock, bool fForceProcessing, CDiskBlockPos* dbp)
 {
-    // Preliminary checks
-    bool checked = CheckBlock(*pblock, state);
-
     {
         LOCK(cs_main);
         bool fRequested = MarkBlockAsReceived(pblock->GetHash());
         fRequested |= fForceProcessing;
-        if (!checked) {
-            return error("%s: CheckBlock FAILED", __func__);
-        }
 
         // Store to disk
         CBlockIndex *pindex = NULL;