Ignore CMPCTBLOCK messages for pruned blocks

Also ignores CMPCTBLOCK announcements that have too little work.  This is to
prevent disk-exhaustion DoS.

src/main.cpp

@@ -5626,8 +5626,8 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
                 std::vector<CInv> vInv(1);
                 vInv[0] = CInv(MSG_BLOCK, cmpctblock.header.GetHash());
                 pfrom->PushMessage(NetMsgType::GETDATA, vInv);
-                return true;
             }
+            return true;
         }
 
         // If we're not close to tip yet, give up and let parallel block fetch work its magic