LPE tool triggers crash when canceling action with tool switch

[Evan Widloski]

Steps to reproduce:

1. Select geometric construction tool > Line Segment

2. Click on canvas to start drawing line segment

3. Select Transform Tool or hit F1

Inkscape then crashes.

I'm using Inkscape 0.92pre2 on Fedora 25.

[su_v (suv-lp)]

Please note that the geometric construction tools are part of the experimental path effects, and not enabled in the final stable release 0.92.0 r15299 (nor in any stable release before that).

[su_v (suv-lp)]

Crash reproduced with lp:inkscape/0.92.x r15315 (build explicitly configured to enable experimental path effects: -DLPE_ENABLE_TEST_EFFECTS -DWITH_LPETOOL) on OS X 10.7.5:
The crash occurs when cancelling drawing the line segment by switching tools. If the line segment has been finished (two clicks), there is no crash when switching tools.

[su_v (suv-lp)]

Based on tests with archived builds, this is not a recent regression:
- reproduced with Inkscape 0.91 (experimental LPEs enabled)
- reproduced with lp:inkscape rev 10795 (oldest archived 64bit trunk build with LPETOOL)
- reproduced with lp:inkscape rev 9760 (oldest archived 32bit trunk build with LPETOOL)

[jazzynico (jazzynico)]

Also reproduced on Xubuntu 16.04, lp:inkscape rev. 15415.