Check jclasses are actually classes in ti_redefine.

There were a few places where we decoded jclasses and cast them to
mirror::Class without actually checking that they are classes first.
This makes us correctly return an INVALID_CLASS error if a non-class
jobject is given.

Test: ./test.py --host -j80

Change-Id: I4cee80726409019ff174333b59a5e4395c069ff6