Do not remove speculative frames in all cases.

If the first frame of an unwind is a totally invalid pc that's not in
any map, a speculative frame is added. Rather than deleting this frame
if no more unwinding is possible, leave it. This fixes a case where
the only frame you get is an invalid one, but the speculative frame
winds up in a shared library or somewhere else and gets removed.

Bug: 120505086

Test: New unit tests to catch this case pass.
Test: Verified original crashing program now emits two backtrace lines.
Change-Id: I088dff21c057386dcdaeb3fc2578b24322683bd0