Bug 460688

==== SUMMARY
Crashes when trying to revert changes after "Time Remapping" alias Speed Ramp.

==== STEPS TO REPRODUCE
[0. Load existing profile.]
1. Apply speed ramp to clip
2. Make some time changes
3. De-select clip and undo changes via CTRL+Z

==== OBSERVED RESULT
Problem signature:
  Problem Event Name:	APPCRASH
  Application Name:	kdenlive.exe
  Application Version:	0.0.0.0
  Application Timestamp:	00000000
  Fault Module Name:	libmlt++-7.dll
  Fault Module Version:	0.0.0.0
  Fault Module Timestamp:	00000000
  Exception Code:	c0000005
  Exception Offset:	000000000000939b
  OS Version:	6.1.7601.2.1.0.256.48
  Locale ID:	1031
  Additional Information 1:	d99f
  Additional Information 2:	d99fe4f86dd04d2d5b3aec01d3dc19f1
  Additional Information 3:	ea16
  Additional Information 4:	ea16c338e91f971974e5362c820ef98e

==== EXPECTERED RESULT
When the very first recorded history/state of the project is reached do nothing even if I though I continue to press CTRL+Z.

==== SOFTWARE/OS VERSIONS
Windows: 7

==== ADDITIONAL INFORMATION
Have the impression that kdenlive tried to go "before" the very first state like "state -1" since I think I already saw the very first state reemerging in the timeline right before the crash while hitting multiple times CTRL+Z.