Coverity 1500722: Warnings in congestion control code

From alert:

** CID 1500722:    (RESOURCE_LEAK)
/src/core/or/congestion_control_common.c: 1349 in congestion_control_build_ext_response()
/src/core/or/congestion_control_common.c: 1349 in congestion_control_build_ext_response()

*** CID 1500722:    (RESOURCE_LEAK)
/src/core/or/congestion_control_common.c: 1349 in congestion_control_build_ext_response()
1343       if (ext) {
1344         trn_extension_free(ext);
1345       } else {
1346         trn_extension_field_free(field);
1347       }
1348       trn_extension_field_cc_free(cc_field);
>>>     CID 1500722:    (RESOURCE_LEAK)
>>>     Variable "field" going out of scope leaks the storage it points to.
1349       return (int)ret;
1350     }
1351     
1352     /** Return true iff the given sendme increment is within the acceptable
1353      * margins. */
1354     bool
/src/core/or/congestion_control_common.c: 1349 in congestion_control_build_ext_response()
1343       if (ext) {
1344         trn_extension_free(ext);
1345       } else {
1346         trn_extension_field_free(field);
1347       }
1348       trn_extension_field_cc_free(cc_field);
>>>     CID 1500722:    (RESOURCE_LEAK)
>>>     Variable "field" going out of scope leaks the storage it points to.
1349       return (int)ret;
1350     }
1351     
1352     /** Return true iff the given sendme increment is within the acceptable
1353      * margins. */
1354     bool