k12: plug a memory leak.

If the two putative number-of-records values don't match (meaning one of
them is presumably the number of records and the other one isn't - we
don't know which is the case), free up the private data structure we
allocated before returning an error.

wiretap/k12.c

@@ -888,6 +888,7 @@ wtap_open_return_val k12_open(wtap *wth, int *err, gchar **err_info) {
                                         K12_FILE_HDR_RECORD_COUNT_1,
                                         pntoh32( header_buffer + K12_FILE_HDR_RECORD_COUNT_2 ),
                                         K12_FILE_HDR_RECORD_COUNT_2 );
+            destroy_k12_file_data(file_data);
             return WTAP_OPEN_ERROR;
         }
     }