If AcceptBlockHeader returns true, pindex will be set.

Assert this instead of checking (and then dref'ing later anyway)
to make sure no one thinks they can change that postcondition of
AcceptBlockHeader..

src/main.cpp

@@ -5218,10 +5218,10 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             }
         }
 
-        if (pindexLast)
+        assert(pindexLast);
         UpdateBlockAvailability(pfrom->GetId(), pindexLast->GetBlockHash());
 
-        if (nCount == MAX_HEADERS_RESULTS && pindexLast && hasNewHeaders) {
+        if (nCount == MAX_HEADERS_RESULTS && hasNewHeaders) {
             // Headers message had its maximum size; the peer may have more headers.
             // TODO: optimize: if pindexLast is an ancestor of chainActive.Tip or pindexBestHeader, continue
             // from there instead.