Fix exception double free in Convex decomposition

[GilesBathgate]

Summary of Changes

Change the order of delete in External_structure_builder.h to avoid a double free in the exception case

Release Management

• Affected package(s): Convex_decomposition_3
• Issue(s) solved (if any): fix convex_decomposition_3: Uncatchable exception causes crash after copying Nef_3 #422
• Feature/Small Feature (if any): bugfix
• License and copyright ownership: Returned to CGAL authors

[sloriot]

Successfully tested in CGAL-6.0-Ic-318

[sloriot]

Seems worth going into 5.5.x, but on the other end the issue is so old that 6.0 is enough

[sloriot]

AFAIS, it does fix the segfault of 422 but not the assertion failing. I'll try to have a look later to see if the fix is easy

[GilesBathgate]

Yes I think issue #422 was about the segfault. Didn't @kintel raise another issue regarding the assertion fail, or do we even expect the nef input provided to be decompose-able?

[sloriot]

I think you are right. There is an isolated vertex that seems to be responsible for not being decomposable. I'll merge the PR as it fixes an issue.