r600/sfn: Don't try to re-use iterators when the set is made empty
It seems with libc++ the iterators are invalidated when the set is emptied during iterating over it.
Fixes: 05fab97b (r600/sfn: Don't try to re-use the iterator when uses is updated)
Closes: #7931 (closed)
Signed-off-by: Gert Wollny gert.wollny@collabora.com