Fix container overflow for compact dex writer

ASAN detected a container overflow regarding string item
deduping. The problem was that Skip(1) did not ensure growth of the
backing buffer. This resulted in occasionally reading one byte past
the end. Also added ensure growth to Seek().

Test: SANITIZE_HOST=address mma dump-oat-boot -k -j64
Bug: 63756964
Change-Id: I5b91f9e6cca7ee37cc304517f4655ba8946b772d