Improve error reporting on DBI large number test failures.

There are other reasons than the original bug that can cause failure,
get some more information to understand what's going on.

libgnucash/backend/dbi/gnc-backend-dbi.cpp

@@ -495,7 +495,7 @@ error_handler<DbType::DBI_MYSQL> (dbi_conn conn, void* user_data)
     if (!dbi_be->connected())
     {
         PINFO ("DBI error: %s\n", msg);
-        PINFO ("Note: GbcDbiSqlConnection not yet initialized. Skipping further error processing.");
+        PINFO ("Note: GncDbiSqlConnection not yet initialized. Skipping further error processing.");
         return;
     }
 
@@ -1205,6 +1205,21 @@ gnc_module_finalize_backend_dbi (void)
 
 /* --------------------------------------------------------- */
 
+static void
+log_failed_field(dbi_result result, const char* fieldname)
+{
+    auto idx = dbi_result_get_field_idx(result, fieldname);
+    if (dbi_result_field_is_null_idx(result, idx))
+        PERR("Result field %s is NULL", fieldname);
+    else
+    {
+        auto type = dbi_result_get_field_type_idx(result, idx);
+        auto attribs = dbi_result_get_field_attribs_idx(result, idx);
+        PERR("Result field %s has type %d and attribs %d",
+             fieldname, type, attribs);
+    }
+}
+
 /** Users discovered a bug in some distributions of libdbi, where if
  * it is compiled on certain versions of gcc with the -ffast-math
  * compiler option it fails to correctly handle saving of 64-bit
@@ -1246,7 +1261,7 @@ dbi_library_test (dbi_conn conn)
     dbi_result_free (result);
     auto locale = gnc_push_locale (LC_NUMERIC, "C");
     result = dbi_conn_query (conn, "SELECT * FROM numtest");
-    if (result == nullptr)
+    if (result == nullptr || !dbi_result_get_numrows(result))
     {
         const char* errmsg;
         dbi_conn_error (conn, &errmsg);
@@ -1259,8 +1274,14 @@ dbi_library_test (dbi_conn conn)
     while (dbi_result_next_row (result))
     {
         resultlonglong = dbi_result_get_longlong (result, "test_int");
+        if (!resultlonglong)
+            log_failed_field(result, "test_int");
         resultulonglong = dbi_result_get_ulonglong (result, "test_unsigned");
+        if (!resultulonglong)
+            log_failed_field(result, "test_unsigned");
         resultdouble = dbi_result_get_double (result, "test_double");
+        if (!resultdouble)
+            log_failed_field(result, "test_double");
     }
     dbi_conn_query (conn, "DROP TABLE numtest");
     gnc_pop_locale (LC_NUMERIC, locale);