convert: check existence of ~/.cvspass before reading it

hgext/convert/cvs.py

@@ -201,7 +201,9 b' class convert_cvs(converter_source):'
 
                 if not passw:
                     passw = "A"
-                    pf = open(os.path.expanduser("~/.cvspass"))
+                    cvspass = os.path.expanduser("~/.cvspass")
+                    if os.path.exists(cvspass):
+                        pf = open(cvspass)
                     for line in pf.read().splitlines():
                         part1, part2 = line.split(' ', 1)
                         if part1 == '/1':