Merge bitcoin#11238: Add assertions before potential null deferences

c001992 Fix potential null dereferences (MeshCollider)

Pull request description:

  Picked up by the static analyzer [Facebook Infer](http://fbinfer.com/) which I was playing around with for another research project. Just adding some asserts before dereferencing potentially null pointers.

Tree-SHA512: 9c01dab2d21bce75c7c7ef867236654ab538318a1fb39f96f09cdd2382a05be1a6b2db0a1169a94168864e82ffeae0686a383db6eba799742bdd89c37ac74397

src/miner.cpp

@@ -136,6 +136,7 @@ std::unique_ptr<CBlockTemplate> BlockAssembler::CreateNewBlock(const CScript& sc
     LOCK2(cs_main, mempool.cs);
 
     CBlockIndex* pindexPrev = chainActive.Tip();
+    assert(pindexPrev != nullptr);
     nHeight = pindexPrev->nHeight + 1;
 
     bool fDIP0003Active_context = nHeight >= chainparams.GetConsensus().DIP0003Height;

src/net_processing.cpp

@@ -337,6 +337,7 @@ bool MarkBlockAsReceived(const uint256& hash) {
     std::map<uint256, std::pair<NodeId, std::list<QueuedBlock>::iterator> >::iterator itInFlight = mapBlocksInFlight.find(hash);
     if (itInFlight != mapBlocksInFlight.end()) {
         CNodeState *state = State(itInFlight->second.first);
+        assert(state != nullptr);
         state->nBlocksInFlightValidHeaders -= itInFlight->second.second->fValidatedHeaders;
         if (state->nBlocksInFlightValidHeaders == 0 && itInFlight->second.second->fValidatedHeaders) {
             // Last validated block on the queue was received.

src/rpc/blockchain.cpp

@@ -1080,6 +1080,7 @@ static void ApplyStats(CCoinsStats &stats, CHashWriter& ss, const uint256& hash,
 static bool GetUTXOStats(CCoinsView *view, CCoinsStats &stats)
 {
     std::unique_ptr<CCoinsViewCursor> pcursor(view->Cursor());
+    assert(pcursor);
 
     CHashWriter ss(SER_GETHASH, PROTOCOL_VERSION);
     stats.hashBlock = pcursor->GetBestBlock();
@@ -1815,6 +1816,8 @@ UniValue getchaintxstats(const JSONRPCRequest& request)
             pindex = chainActive.Tip();
         }
     }
+
+    assert(pindex != nullptr);
 
     if (blockcount < 1 || blockcount >= pindex->nHeight) {
         throw JSONRPCError(RPC_INVALID_PARAMETER, "Invalid block count: should be between 1 and the block's height");

src/validation.cpp

@@ -290,6 +290,8 @@ bool CheckSequenceLocks(const CTransaction &tx, int flags, LockPoints* lp, bool
     AssertLockHeld(mempool.cs);
 
     CBlockIndex* tip = chainActive.Tip();
+    assert(tip != nullptr);
+
     CBlockIndex index;
     index.pprev = tip;
     // CheckSequenceLocks() uses chainActive.Height()+1 to evaluate