Add check that TypeIndex is valid in StringByTypeIdx.

StringByTypeIdx should fail gracefully if given a bad TypeIndex. This
adds a check that the TypeIndex is valid before getting its TypeId.

This fixes a regression that removed this check when it was refactored
in this CL: https://android-review.googlesource.com/#/c/243493/

Bug: 37287051
Test: mm -j31 test-art-host-gtest-dex_file_test
Change-Id: Ib68cb8135011f5f30335251583e181b089982754