bitbake: cooker: Avoid error if siggen wasn't setup

In some cases we'd never have setup a siggen so don't error in that case.

(Bitbake rev: bbaaf2cf7b5a9339d3790610e622020c19d52f5a)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

bitbake/lib/bb/cooker.py

@@ -1745,7 +1745,8 @@ class BBCooker:
     def post_serve(self):
         self.shutdown(force=True)
         prserv.serv.auto_shutdown()
-        bb.parse.siggen.exit()
+        if hasattr(bb.parse, "siggen"):
+            bb.parse.siggen.exit()
         if self.hashserv:
             self.hashserv.process.terminate()
             self.hashserv.process.join()