posix: replace superfluous pass statement with explicit return

posix: replace superfluous pass statement with explicit return

--- a/mercurial/posix.py	Sat Sep 30 07:45:51 2017 -0400
+++ b/mercurial/posix.py	Sat Sep 30 07:45:18 2017 -0400
@@ -300,7 +300,7 @@
 def checkosfilename(path):
     '''Check that the base-relative path is a valid filename on this platform.
     Returns None if the path is ok, or a UI string describing the problem.'''
-    pass # on posix platforms, every path is ok
+    return None # on posix platforms, every path is ok
 
 def setbinary(fd):
     pass