Fixed compilation failure using clang

Both aria and myisam storage engines feature a logic path in
thr_find_all_keys that leads to undefined behaviour by bypassing the
initialization code of variables after my_thread_init().

By refactoring the nested logic into a separate function, this problem
is resolved.

include/myisam.h

@@ -330,7 +330,8 @@ typedef struct st_sort_info
   my_off_t filelength, dupp, buff_length;
   ha_rows max_records;
   uint current_key, total_keys;
-  uint got_error, threads_running;
+  volatile uint got_error;
+  uint threads_running;
   myf myf_rw;
   enum data_file_type new_data_file_type;
 } MI_SORT_INFO;