libopenmpt: update to 0.5.12.

### libopenmpt 0.5.12 (2021-10-04)

*  [**Sec**] Possible crash when loading malformed MDL files. (r15603)

*  [**Bug**] Fixed various undefined behaviour found with ubsan.

*  Seeking with sample sync sometimes didn't compute the correct sample
    position with pingpong-looped samples.
*  IT: Tremor command I11 erroneously behaved like I00 (use previous
    parameter) unless IT Old Effects were enabled.
*  PTM: Panning was translated wrong in some edge cases.
*  IMF / PTM: Note Slide commands were sometimes slightly off.
*  OKT: Better support for fine note slides.
*  DBM: Echo enable effect parameter range checks were incorrect.
*  XM: Sample texts in XMs made with MadTracker are now also decoded using
    Windows-1252 encoding.

*  in_openmpt: Song metadata is no longer reverted when viewing file info.

(fcambus)