Add assertion to guide static analyzers. Clang Static Analyzer needs this guidance.

src/rpc/mining.cpp

@@ -525,6 +525,7 @@ UniValue getblocktemplate(const JSONRPCRequest& request)
         // Need to update only after we know CreateNewBlock succeeded
         pindexPrev = pindexPrevNew;
     }
+    assert(pindexPrev);
     CBlock* pblock = &pblocktemplate->block; // pointer for convenience
     const Consensus::Params& consensusParams = Params().GetConsensus();