odbc_result/connection: better reference accounting (#731)

---------

Co-authored-by: Simon P. Couch <simonpatrickcouch@gmail.com>

NEWS.md

@@ -1,5 +1,8 @@
 # odbc (development version)
 
+* freetds: Fix regression when executing multiple queries
+  (@detule, #731).
+
 * `dbAppendTable()` Improve performance by checking existence once (#691).
 
 * Teradata: Fix usage of `exact` argument in internal methods

src/odbc_connection.h

@@ -18,6 +18,7 @@ class odbc_result;
 
 class odbc_connection {
 public:
+  friend odbc_result;
   odbc_connection(
       std::string connection_string,
       std::string timezone = "UTC",

src/odbc_result.cpp

@@ -214,6 +214,12 @@ odbc_result::~odbc_result() {
     try {
       c_->set_current_result(nullptr);
     } catch (...) {
+      // SQLCancel may throw an error.
+      // Regardless, as this object is
+      // getting destroyed, we need to
+      // make sure the connection is not
+      // holding onto an invalid reference
+      c_->current_result_ = nullptr;
     };
   }
 }