Don't visit invalid Dex-cache slots

We were incorrectly visiting invalid dex-cache slots during
VisitReflectiveTargets. This didn't cause any real problems but it is
neither expected nor useful.

Test: ./test.py --host
Bug: 134162467
Change-Id: I2dedbcff66bfb4fa093d23ea79e1054aa5a9636b
2 files changed