IAccessible2: revert some bounds-checking changes again

...that were introduced with c23ab5eaed57cc3fb5860e26c591c73c5b22687b "Integrate
branch of IAccessible2" but broke JunitTest_sc_unoapi, which expects calls to
getCharacterAttributes and getIndexAtPoint to actually fail for out-of-bounds
arguments.  The above commit does not make it obvious why those changes were
actually made.

Change-Id: I3492c3e6a49d68885df1206e9368f4483c0dc4f9