Fix non-existing pattern error

[hillerstorm]

Tested against remember_by_stalker_of_fyllecell303_.xm.

The pattern at the last song position in player.xm.songpats is 26 which
doesn't exist in player.xm.patterns.
This causes p in the following line to return undefined resulting in
multiple errors being logged:

var p = player.xm.patterns[player.cur_pat];
var r = p[player.cur_row];

To jump to the last pattern instantly set a breakpoint in xm.js:play(),
run the following code in the debugger and then resume:

player.cur_songpos=29;player.cur_pat=25;player.cur_row=0

[a1k0n]

Ah, that's annoying. Thanks for adding, and nice tests! I am gonna have dinner and take a closer look.

My only concern is that it's an infinite loop if the song has length 0, but it might bail out earlier in that case. Not sure.

[a1k0n]

This makes sense and looks good. Would you mind adding a test for the song_looppos logic?

[hillerstorm]

Sure, I'll add a test or two and rebase

[a1k0n]

Awesome. Thanks!