Merge pull request #759 from kubo/segv-by-freed-result

Fix SEGV when `wrapper->result` is used after it is freed.

Author: sodabrew

ext/mysql2/result.c

@@ -920,6 +920,9 @@ static VALUE rb_mysql_result_each(int argc, VALUE * argv, VALUE self) {
     wrapper->numberOfRows = wrapper->stmt_wrapper ? mysql_stmt_num_rows(wrapper->stmt_wrapper->stmt) : mysql_num_rows(wrapper->result);
     wrapper->rows = rb_ary_new2(wrapper->numberOfRows);
   } else if (wrapper->rows && !cacheRows) {
+    if (wrapper->resultFreed) {
+      rb_raise(cMysql2Error, "Result set has already been freed");
+    }
     mysql_data_seek(wrapper->result, 0);
     wrapper->lastRowProcessed = 0;
     wrapper->rows = rb_ary_new2(wrapper->numberOfRows);