Fix debug assert post MDEV-10306

tmp variable now points to str->ptr() buffer, not tmp_value buffer.
Comparing pointers otherwise can lead to false assertion errors as we
don't know where buffers are allocated in respect to each other.

sql/item_strfunc.cc

@@ -1171,7 +1171,7 @@ String *Item_func_reverse::val_str(String *str)
       if ((l= my_ismbchar(res->charset(),ptr,end)))
       {
         tmp-= l;
-        DBUG_ASSERT(tmp >= tmp_value.ptr());
+        DBUG_ASSERT(tmp >= str->ptr());
         memcpy(tmp,ptr,l);
         ptr+= l;
       }