bitbake: runqueue: Ensure handler does not recurse

Failures on the autobuilder look like this handler is recursing. That
shouldn't be possible but it doesn't hurt to code as such.

(Bitbake rev: e39e85803cbe1ef9413a118868c19087c0546d01)

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

bitbake/lib/bb/runqueue.py

@@ -930,7 +930,7 @@ class RunQueue:
                     self.finish_runqueue(True)
             except OSError:
                 pid = False
-        if callable(self.oldsigchld):
+        if callable(self.oldsigchld) and self.oldsigchld != self.sigchild_exception:
             self.oldsigchld(*args, **kwargs)
 
     def start_worker(self):