binlog_cache_data: use the correct cache size in reset()

sql/log_cache.h

@@ -94,9 +94,9 @@ class binlog_cache_data
   void reset()
   {
     bool cache_was_empty= empty();
+    my_off_t trunc_len= MY_MIN(CACHE_FILE_TRUNC_SIZE, cache_log.buffer_length);
     bool truncate_file= (cache_log.file != -1 &&
-                         my_b_write_tell(&cache_log) >
-                         MY_MIN(CACHE_FILE_TRUNC_SIZE, binlog_stmt_cache_size));
+                         my_b_write_tell(&cache_log) > trunc_len);
     truncate(0,1);                              // Forget what's in cache
     checksum_opt= !precompute_checksums ? BINLOG_CHECKSUM_ALG_OFF :
       (enum_binlog_checksum_alg)binlog_checksum_options;