don't flush dbug buffers for every assert

only do it before crashing.

Otherwise it introduces a noticable slowdown and,
most importantly, changes errno, when the caller
doesn't expect it.

include/my_dbug.h

@@ -102,8 +102,8 @@ extern  const char* _db_get_func_(void);
 #define DBUG_END()  _db_end_ ()
 #define DBUG_LOCK_FILE _db_lock_file_()
 #define DBUG_UNLOCK_FILE _db_unlock_file_()
-#define DBUG_ASSERT(A) ( _db_flush_(), assert(A) )
-#define DBUG_SLOW_ASSERT(A) ( _db_flush_(), assert(A) )
+#define DBUG_ASSERT(A) do { if (!(A)) { _db_flush_(); assert(A); }} while (0)
+#define DBUG_SLOW_ASSERT(A) DBUG_ASSERT(A)
 #define DBUG_ASSERT_EXISTS
 #define DBUG_EXPLAIN(buf,len) _db_explain_(0, (buf),(len))
 #define DBUG_EXPLAIN_INITIAL(buf,len) _db_explain_init_((buf),(len))