[Driver] Fix a segfault in signal handlers

[labath]

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.

lldb/trunk/tools/driver/Driver.cpp

sigint_handler (int signo)
   }
     
   exit (signo);
 }
 
 void
 sigtstp_handler (int signo)
 {
+    if (g_driver)
         g_driver->GetDebugger().SaveInputTerminalState();
+
     signal (signo, SIG_DFL);
     kill (getpid(), signo);
     signal (signo, sigtstp_handler);
 }
 
 void
 sigcont_handler (int signo)
 {
+    if (g_driver)
         g_driver->GetDebugger().RestoreInputTerminalState();
+
     signal (signo, SIG_DFL);
     kill (getpid(), signo);
     signal (signo, sigcont_handler);
 }
 
 int
 #ifdef WIN32
 wmain(int argc, wchar_t const *wargv[])