[Core] Restore CheckBlock checks before previous block check

in ProcessNewBlock (as it was before nStakeModifierV2) to avoid possible
syncing issues.

src/main.cpp

@@ -5169,6 +5169,15 @@ void CBlockIndex::BuildSkip()
 
 bool ProcessNewBlock(CValidationState& state, CNode* pfrom, CBlock* pblock, CDiskBlockPos* dbp)
 {
+    // Preliminary checks
+    int64_t nStartTime = GetTimeMillis();
+
+    // check block
+    bool checked = CheckBlock(*pblock, state);
+
+    if (!CheckBlockSignature(*pblock))
+        return error("%s : bad proof-of-stake block signature", __func__);
+
     if (pblock->GetHash() != Params().HashGenesisBlock() && pfrom != NULL) {
         //if we get this far, check if the prev block is our prev block, if not then request sync and return false
         BlockMap::iterator mi = mapBlockIndex.find(pblock->hashPrevBlock);
@@ -5178,15 +5187,6 @@ bool ProcessNewBlock(CValidationState& state, CNode* pfrom, CBlock* pblock, CDis
         }
     }
 
-    // Preliminary checks
-    int64_t nStartTime = GetTimeMillis();
-
-    // check block
-    bool checked = CheckBlock(*pblock, state);
-
-    if (!CheckBlockSignature(*pblock))
-        return error("ProcessNewBlock() : bad proof-of-stake block signature");
-
     {
         LOCK(cs_main);