Add check for reverse getBlockHeaders

eth/p2p/blockchain_utils.nim

@@ -10,7 +10,8 @@ proc getBlockHeaders*(db: AbstractChainDB,
   var foundBlock: BlockHeader
   if db.getBlockHeader(req.startBlock, foundBlock):
     result.add foundBlock
-
+    # Quick sanity check for lower bounds, code should be safe though without.
+    if not req.reverse or (foundBlock.blockNumber >= (req.skip + 1).toBlockNumber):
       while uint64(result.len) < req.maxResults:
         if not req.reverse:
           if not db.getSuccessorHeader(foundBlock, foundBlock, req.skip):
@@ -20,6 +21,7 @@ proc getBlockHeaders*(db: AbstractChainDB,
             break
         result.add foundBlock
 
+
 template fetcher*(fetcherName, fetchingFunc, InputType, ResultType: untyped) =
   proc fetcherName*(db: AbstractChainDB,
                     lookups: openarray[InputType]): seq[ResultType] {.gcsafe.} =