ProcessMinidump: Suppress reporting stop for signal '0'

Summary:
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.

Reviewers: labath, clayborg, jfb

Reviewed By: labath, clayborg

Subscribers: dexonsmith, lldb-commits

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D68096

llvm-svn: 375244