Make GetAncestor more robust

This check for pskip != NULL was introduced in bitcoin#5927 for 0.12.
It is in general safer and allows GetAncestor to be used in more places, specifically in the mining tests for the backport of BIP 68 to 0.11.

src/chain.cpp

@@ -82,9 +82,10 @@ CBlockIndex* CBlockIndex::GetAncestor(int height)
     while (heightWalk > height) {
         int heightSkip = GetSkipHeight(heightWalk);
         int heightSkipPrev = GetSkipHeight(heightWalk - 1);
-        if (heightSkip == height ||
-            (heightSkip > height && !(heightSkipPrev < heightSkip - 2 &&
-                                      heightSkipPrev >= height))) {
+         if (pindexWalk->pskip != NULL &&
+             (heightSkip == height ||
+              (heightSkip > height && !(heightSkipPrev < heightSkip - 2 &&
+                                        heightSkipPrev >= height)))) {
             // Only follow pskip if pprev->pskip isn't better than pskip->pprev.
             pindexWalk = pindexWalk->pskip;
             heightWalk = heightSkip;