Small fix for msvcrt locking code

[Matt Lavin]

This should fix many warnings being reported to the console when using 
msvrt locking.  This is my very first time looking at or modifying the 
source of bzr, so I could be way off base.  I couldn't figure out how to 
create a nice patch file that I think I've seen on this list but here is 
the diff

--- bzrlib\lock.py
+++ bzrlib\lock.py
@@ -168,6 +168,8 @@
                 LOCK_NB = 4
                 def unlock(self):
                     _msvc_unlock(self.f)
+                    self.f.close()
+                    self.f = None


             class _msvc_ReadLock(_msvc_FileLock):