config: fix consider_preparse with missing argument to -p

This is only required after/with 415899d4 - otherwise argparse ensures
there is an argument already.

src/_pytest/config/__init__.py

@@ -481,7 +481,10 @@ class PytestPluginManager(PluginManager):
             i += 1
             if isinstance(opt, six.string_types):
                 if opt == "-p":
-                    parg = args[i]
+                    try:
+                        parg = args[i]
+                    except IndexError:
+                        return
                     i += 1
                 elif opt.startswith("-p"):
                     parg = opt[2:]