ffmsindex: do not report an error when no options are supplied
Merge pull request #169 from kodabb/returnvalue

ffmsindex: do not report an error when no options are supplied
Merge branch 'master' into cplugin_master
c_plugin: Apply the plugin-related changes from master @ 12b6c1ae.

Not applying this earlier caused the rffmode regression fix to
cause the same problem it was intended to solve.

There are some differences because there were portions of the
change that already existed in the C-plugin.
avxsynth: Synchronize plugin with upstream master.