patman based project without valid patch: will crash

If a project is linked to a missing patch file; it terminates LMMS when it is just about to play that first note.

Ways to reproduce:

1. create project with valid pat with some notes
2. Save the mmpz file.
3. Rename, delete, unlink or move away that patch file.
4. Play the project.

Another way:

1. `touch xyz.pat` (create a random, invalid pat file, without its headers).
2. Load that .pat into the instrument.

It terminates the application.

Reproduce the crash with a project whose pat file was moved or deleted, and with an invalid xyz.pat created using touch. Trace the patman loading and first-note playback path; done means LMMS remains running for both missing and invalid patch files.

Written by the indexing model from the issue text.