[Driver] Fix a segfault in signal handlers
[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

git-svn-id: 4c4cc70b1ef44ba2b7963015e681894188cea27e@265958 91177308-0d34-0410-b5e6-96231b3b80d8

apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,llvm-mirror/lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,apple/swift-lldb,llvm-mirror/lldb,llvm-mirror/lldb,llvm-mirror/lldb