fix #25579 : possible buffer overflow

[AntonioBL]

No description provided.

[lasconic]

Why are you doing this here? It seems that this shouldn't happen right? If you encounter a file that crash MuseScore because of this, we should fix the file reading. If we are encountering a crash during layout in this code, then we should fix the layout code. By fixing the bug in this place, we would be hiding the actual cause of the bug and never be able to fix it.

[AntonioBL]

With address sanitizer switched on (at compilation time), I cannot use MuseScore because of this buffer overflow, since it now happens also for new files. See http://musescore.org/en/node/25579#comment-105021
This was just a work-around to skip this problem and the behavior should be analogous to the one that happens in MuseScore without address sanitizer, since the undefined behavior would most probably give something random or 0.