As a side-effect also prevents a segfault possible by providing just 3 points.
[Sketcher] Clarify that mults might not be followed
Some examples when this may be ignored include:
1. A number higher than 3 is given (because cubic B-splines are created)
2. If the knots just before and just after have multiplicities >=3, the piece
between these two is fully continuous, and this (middles) point will only be
constrained with point-on-object.
This commit is part of a project funded by the Open Toolchain Foundation under the title "Open Toolchain Foundation - Curve drawing tool in Sketcher Workbench"
4b61edf470