[agroal#334] Throw an error when config-get refers to a non existin…

…g key

This commit fixes the problem that arise when the user specifies
a global key (i.e., one that does not exist in the section
[pgagroal]) to the subcommand `config-get`.
If the key is not known, an error is thrown.

Close agroal#334

src/libpgagroal/configuration.c

@@ -3513,6 +3513,11 @@ pgagroal_write_config_value(char* buffer, char* config_key)
       {
          return to_bool(buffer, config->track_prepared_statements);
       }
+      else
+      {
+         goto error;
+      }
+
    } // end of global configuration settings
    else
    {