calculate estimated result size for stats queries

output size is always zero here until we calculate the final result but it would be good to cancel runaway stats queries as well, ex. stats with high cardinality column set

src/Query.cc

@@ -966,6 +966,17 @@ bool Query::processDataset(void *data)
                 if( is_new ) {
                     _current_line++;
                     _sorter.insert( data, _limit+_offset );
+
+                    // make sure we don't create too many aggregation entries. The size is only a rough estimation
+                    // from the last entry mulitplies with the number of entries.
+                    size_t rowsize = 0;
+                    for (_stats_group_spec_t::iterator iit = groupspec.begin(); iit != groupspec.end(); ++iit)
+                        rowsize += sizeof(char*) * strlen((*iit).c_str());
+                    if (_sorter.size() * rowsize > g_max_response_size) {
+                        logger(LG_INFO, "Maximum response size of %d bytes exceeded!", g_max_response_size);
+                        _output->setError(RESPONSE_CODE_LIMIT_EXCEEDED, "Maximum response size of %d reached", g_max_response_size);
+                        return false;
+                    }
                 }
             }
             else