Clear exception before calling ValidateSuperClassDescriptors

Since ValidateSuperClassDescriptors may also cause an exception, we
can't have any stale exceptions from EnsureInitialized otherwise there
may be aborts

TODO: Figure out how to add a test.

Bug: 131855081
Bug: 132690194
Test: test-art-host

(cherry picked from commit 422193f858eb537f4f63c286498bb2d2d0821539)

Merged-In: Ia56af1ff497d30539a42a772979320df699b658a
Change-Id: I6be5c184f52593d11927f7185e11dc868cc1724f