fix #106146 Crash when pressing LEFT in note entry mode

[heuchi]

https://musescore.org/en/node/106146

[lasconic]

The else is not useful anymore then.
Also, it's probably a good case to use the new toSegment(), toChord() everywhere in the function https://github.com/musescore/MuseScore/blob/master/mscore3.txt#L70

[heuchi]

Updated to follow new guide lines and removed the else branch.
I added a Q_ASSERT to make sure s has been assigned a valid pointer.

[heuchi]

The main reason for the crash is the Q_ASSERT statement in Element.isChordRest() in file libmscore/element.h:616

bool isChordRest() const { Q_ASSERT(type() != Element::Type::SEGMENT);
      return type() == Element::Type::REST || type() == Element::Type::CHORD
      || type() == Element::Type::REPEAT_MEASURE; }

I don't quite understand why we disallow a call to isChordRest() for Segment pointers.

EDIT: Just noticed there is isChordRest1()...
Going to update this PR to use that...

[wschweer]

There was aname conflict for isChordRest(); in old code it was used to check Segments (and the segment type). Current code use isChordRest() to test the element type. To find the old uses i added an ASSERT. For legal uses of isChordRest() on segments you have to use isChordRest1(). This is a hack and isChordRest1() will be replaced by isChordRest() and the assert removed.