Convert the -a option in "image lookup" to a address-expression, and pass the context
in the -a address expression lookup in "image list" so that it actually works.
Fix buffer underrun (invalid read) triggered during diagnostic rendering.  The test would overflow when computing '0 - 1'.

I don't have a good testcase for this that does not depend on system headers.
It did not trigger with preprocessed output, and I had trouble reducing the example.

Fixes <rdar://problem/13324594>.

Thanks to Michael Greiner for reporting this issue.