EMI: Fix save/restore when compiled without MP3 support

Saving and restoring the emisound state fails when residualvm
is compiled without libmad MP3 support. This patch makes
openSound() return successfully if the stream is valid but MP3
support is not available.
EMI: Fix fullscreen toggle in main menu

When the main menu is initially opened it will call ImPushState() before
any music is playing. This results in a NULL entry on EMISound's state
stack. saveState/restoreState must be able to deal with this situation
because they are used when toggling fullscreen.