win32: win32console.GetStdHandle() can return None

When the Mercurial Python libraries are used within a Windows application with
no console, there is no stderr file handle.

mercurial/win32.py

@@ -170,6 +170,8 b' def termwidth():'
     try:
         # Query stderr to avoid problems with redirections
         screenbuf = win32console.GetStdHandle(win32console.STD_ERROR_HANDLE)
+        if screenbuf is None:
+            return 79
         try:
             window = screenbuf.GetConsoleScreenBufferInfo()['Window']
             width = window.Right - window.Left