jhbuild wrapper: Don't crash if we can't start the shell

If start_shell throws an exception for whatever reason, the finally
handler will run before shell has been set, leading to a "referencing
an undefined local variable" error.

src/gnome-shell-jhbuild.in

@@ -111,6 +111,7 @@ def run_shell():
     if options.verbose:
         print "Starting shell"
 
+    shell = None
     try:
         shell = start_shell()