Fixed compilation failure on MacOSX

Due to a hack that has propagated to the maria storage engine, undefined
behaviour would result 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

@@ -331,7 +331,8 @@ 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;

storage/maria/maria_def.h

@@ -67,7 +67,8 @@ typedef struct st_maria_sort_info
   pgcache_page_no_t page;
   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, org_data_file_type;
 } MARIA_SORT_INFO;