The minidump exception stream can report an exception record with
signal 0. If we try to create a stop reason with signal zero, processing
of the stop event won't find anything, and the debugger will hang.
So, simply early-out of RefreshStateAfterStop in this case.
Also set the UnixSignals object in DoLoadCore as is done for
ProcessElfCore.
I think you can create this unconditionally.