Fix problem with no arguments

This is currently getting called with no arguments, and requires (as
stands) 2.  I'd handled the lack of 1 argument previously, but as of
getting further along the second argument is missing.

docs/courier-to-mailman3.md

@@ -34,4 +34,5 @@ This is our current version trying to communicate between courier and mailman3.
   * Catch exception, log based on that?
   * Hm, go with bare "Exception"...  The catch wasn't what was failing.
   * Let's try that instead?
-* Oh man...  Variable not properly defined (or more derferenced.)
+* Oh man...  Variable not properly defined (or more derferenced.)
+* OK, it's failing further along, so maybe this works?

source/courier-to-mailman3.py

@@ -50,7 +50,8 @@ try:
         # for qmail command docs and supplied environment variables.
         # We need to replace "1" with an empty string, as qmail only supports EXT,
         # EXT2, EXT3, EXT4.
-        arg_ext = sys.argv[2] if sys.argv[2] != "1" else ""
+        arg_ext_tmp = sys.argv[2] if len(sys.argv) > 2 else "1"
+        arg_ext = arg_ext_tmp if arg_ext_tmp != "1" else ""
         lmtp.sendmail(
             os.environ['SENDER'],
             os.environ['EXT' + arg_ext] + "@" + os.environ['HOST'],