chain: Add assertion in case of missing records in index db

src/chain.cpp

@@ -90,6 +90,7 @@ CBlockIndex* CBlockIndex::GetAncestor(int height)
             pindexWalk = pindexWalk->pskip;
             heightWalk = heightSkip;
         } else {
+            assert(pindexWalk->pprev);
             pindexWalk = pindexWalk->pprev;
             heightWalk--;
         }