Make PruneBlockIndexCandidates safer

Author: sipa

src/main.cpp

@@ -1954,9 +1954,11 @@ static void PruneBlockIndexCandidates() {
     // Note that we can't delete the current block itself, as we may need to return to it later in case a
     // reorganization to a better block fails.
     std::set<CBlockIndex*, CBlockIndexWorkComparator>::iterator it = setBlockIndexCandidates.begin();
-    while (setBlockIndexCandidates.value_comp()(*it, chainActive.Tip())) {
+    while (it != setBlockIndexCandidates.end() && setBlockIndexCandidates.value_comp()(*it, chainActive.Tip())) {
         setBlockIndexCandidates.erase(it++);
     }
+    // Either the current tip or a successor of it we're working towards is left in setBlockIndexCandidates.
+    assert(!setBlockIndexCandidates.empty());
 }
 
 // Try to make some progress towards making pindexMostWork the active block.
@@ -2007,8 +2009,6 @@ static bool ActivateBestChainStep(CValidationState &state, CBlockIndex *pindexMo
             }
         } else {
             PruneBlockIndexCandidates();
-            // Either the current tip or a successor of it we're working towards is left in setBlockIndexCandidates.
-            assert(!setBlockIndexCandidates.empty());
             if (!pindexOldTip || chainActive.Tip()->nChainWork > pindexOldTip->nChainWork) {
                 // We're in a better position than we were. Return temporarily to release the lock.
                 fContinue = false;