Added code to prevent "administrative stop" from overwriting a real stop reason.

Note this code path should not happen - it implies a bug in another part of
the code.  For the thread to receive the stop signal as it is handled, the
and for it to already have a stop reason, it implies the kernel was able to
tell the thread that it stopped while it was stopped.  More likely this
seems to indicate a bug where an actual thread start was not getting correctly
logged.  If it does get hit, we'll want to understand the sequence to figure
out if it is truly legitimate or if it implies another bug.

llvm-svn: 227916