fix #273159: Crash when pasting note with receiving tie on a new score

[dmitrio95]

This is a fix to this issue which happens when trying to paste (or even load from a file) tied notes if a note has sufficiently large track number specified. In the example shown in the issue description something like this is pasted to the score:

<!-- Staff, Chord, etc. -->
<Note>
  <track>4</track>
  <endSpanner id="2"/>
  <!-- other properties -->
  </Note>
<!-- end of Chord, Staff, etc. -->

so the Note was considered to be at the 2nd staff which may eventually not exist in the destination score. Thus staff() calls for the note return a null pointer, and the absence of check for it leads to its dereferencing and a crash of the program. This patch adds the necessary nullpointer checks which prevents program crashes in such cases.