Throw exception if slot has invalid offset

Previously the process would crash, which is OK, but complicates testing.

Test: cts-tradefed run cts --module CtsContentTestCases
      --test android.content.cts.ContentProviderCursorWindowTest
Bug: 34128677

Change-Id: I5b50982d77ec65c442fbb973d14c85a5c29c43c7