[Driver] Fix a segfault in signal handlers

Summary:
If we recieve a SIGCONT or SIGTSTP, while the driver is shutting down (which, sometimes, we do,
for reasons which are not completely clear to me), we would crash to due a null pointer
dereference. Guard against this situation.

Reviewers: clayborg

Subscribers: lldb-commits

Differential Revision: http://reviews.llvm.org/D18965

llvm-svn: 265958