Fix incorrect usage of begin() when genesis block is requested in "pr…

…otx diff" (#2699)

* Fix incorrect usage of begin() when genesis block is requested in "protx diff"

.begin() on mapBlockIndex does NOT return the genesis block, but just the
block with lowest hash.

The fix is to use chainActive[0] to get the genesis block.

* Update src/evo/simplifiedmns.cpp

Co-Authored-By: codablock <ablock84@gmail.com>

src/evo/simplifiedmns.cpp

@@ -119,25 +119,27 @@ bool BuildSimplifiedMNListDiff(const uint256& baseBlockHash, const uint256& bloc
     AssertLockHeld(cs_main);
     mnListDiffRet = CSimplifiedMNListDiff();
 
-    BlockMap::iterator baseBlockIt = mapBlockIndex.begin();
+    const CBlockIndex* baseBlockIndex = chainActive.Genesis();
     if (!baseBlockHash.IsNull()) {
-        baseBlockIt = mapBlockIndex.find(baseBlockHash);
+        auto it = mapBlockIndex.find(baseBlockHash);
+        if (it == mapBlockIndex.end()) {
+            errorRet = strprintf("block %s not found", baseBlockHash.ToString());
+            return false;
+        }
+        baseBlockIndex = it->second;
     }
     auto blockIt = mapBlockIndex.find(blockHash);
-    if (baseBlockIt == mapBlockIndex.end()) {
-        errorRet = strprintf("block %s not found", baseBlockHash.ToString());
-        return false;
-    }
     if (blockIt == mapBlockIndex.end()) {
         errorRet = strprintf("block %s not found", blockHash.ToString());
         return false;
     }
+    const CBlockIndex* blockIndex = blockIt->second;
 
-    if (!chainActive.Contains(baseBlockIt->second) || !chainActive.Contains(blockIt->second)) {
+    if (!chainActive.Contains(baseBlockIndex) || !chainActive.Contains(blockIndex)) {
         errorRet = strprintf("block %s and %s are not in the same chain", baseBlockHash.ToString(), blockHash.ToString());
         return false;
     }
-    if (baseBlockIt->second->nHeight > blockIt->second->nHeight) {
+    if (baseBlockIndex->nHeight > blockIndex->nHeight) {
         errorRet = strprintf("base block %s is higher then block %s", baseBlockHash.ToString(), blockHash.ToString());
         return false;
     }
@@ -150,7 +152,7 @@ bool BuildSimplifiedMNListDiff(const uint256& baseBlockHash, const uint256& bloc
 
     // TODO store coinbase TX in CBlockIndex
     CBlock block;
-    if (!ReadBlockFromDisk(block, blockIt->second, Params().GetConsensus())) {
+    if (!ReadBlockFromDisk(block, blockIndex, Params().GetConsensus())) {
         errorRet = strprintf("failed to read block %s from disk", blockHash.ToString());
         return false;
     }