Fixed lock during block downloading.

src/primitives/block.cpp

@@ -21,8 +21,9 @@ static void cn_get_block_hash_by_height(uint64_t seed_height, char cnHash[32])
 {
     CBlockIndex* pblockindex = chainActive[seed_height];
     if (pblockindex == NULL) {
-        AssertLockHeld(cs_main);
-        // This will only happens during initial block download.
+        // This will happen during initial block downloading, or when we
+        // are out of sync by more than at least SEEDHASH_EPOCH_BLOCKS blocks.
+        LOCK(cs_main);
         pblockindex = mapBlockSeedHeight.find(seed_height)->second;
     }
     if (pblockindex == NULL) {