For now, ignore duplicate sceMpegInit()/Finish()'s.

Need to properly handle module unload for this to work right.  I was worried some games would call it until it returned an error or something silly like that, but I haven't actually seen any games that depend on this returning an already init error.  I think it's safer to ignore it for now.

Fixes #2364, or at least, I think it will based on the log.

-[Unknown]