ART: Fix UTF test and monitor pool old chunks

Zero-initialize buffers in utf_test. UTF conversion does not
append a zero terminator, but the test uses strlen to check the
byte count.

Move to unique_ptr for storing old monitor-pool chunk data to
fix a leak.

Bug: 27156726
Change-Id: I14424c6f98cf47beab6243f83a335bd6a682c638
3 files changed