Fixed compilarion failure on 32bit systems

Compile time assertion "sizeof(struct st_irem) % sizeof(double) == 0" started
to fail on 32bit systems after my_thread_id was changed from ulong to int64.

Fixed by added padding to struct st_irem on 32bit systems.

mysys/safemalloc.c

@@ -55,6 +55,9 @@ struct st_irem
   struct st_irem *next;        /* Linked list of structures       */
   struct st_irem *prev;        /* Other link                      */
   size_t datasize;             /* Size requested                  */
+#if SIZEOF_SIZE_T == 4
+  size_t pad;                  /* Compensate 32bit datasize */
+#endif
 #ifdef HAVE_BACKTRACE
   void *frame[SF_REMEMBER_FRAMES]; /* call stack                  */
 #endif