Fix potential NPD introduced in b297426c

See https://github.com/bitcoin/bitcoin/pull/10290#discussion_r113954232
for more info.

src/validation.cpp

@@ -2538,7 +2538,7 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
     }
 
     int nStopAtHeight = GetArg("-stopatheight", DEFAULT_STOPATHEIGHT);
-    if (nStopAtHeight && pindexNewTip->nHeight >= nStopAtHeight) StartShutdown();
+    if (nStopAtHeight && pindexNewTip && pindexNewTip->nHeight >= nStopAtHeight) StartShutdown();
 
     return true;
 }