Crash if part's left boundary is expanded to the left #842

Scenario:

• New song
• Create part with arbitrary length at bar LADSPA plugins with wrong number of inputs crash MusE [sf#838151] #1 and draw a note, starting at the very left, at beat 1.
• With the same draw tool, try to expand the left boundary of the part to the left.
  Boooooom bang bye!

Of course this is not a major issue (just don't expand bar1 to the left), but a DAW should not crash that willful. As said before, it is OK for me to be the tester ;-)

And no, I don't find the mistakes out of malice. I wanted to add a count-in to the left of the song because I do not like the restrictive metronome. I just failed to move the entire song one bar to the right before.