_LockProcess: fix _poll returncode

We don't want to return the returncode directly from the
subprocess. Return self.returncode instead.
diff --git a/pym/_emerge/AsynchronousLock.py b/pym/_emerge/AsynchronousLock.py
index ec02364..637ba73 100644
--- a/pym/_emerge/AsynchronousLock.py
+++ b/pym/_emerge/AsynchronousLock.py
@@ -239,7 +239,7 @@
 
 	def _poll(self):
 		if self._proc is not None:
-			return self._proc.poll()
+			self._proc.poll()
 		return self.returncode
 
 	def _wait(self):