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Fix T79754: Boundary Brush crashing with symmetry enabled

When creating the boundaries for the symmetry passess, it could happen
that a symmetrical vertex is not found inside the radius of the brush.
In that case, the function to initialize a new boundary was called with
index -1 (BOUNDARY_VERTEX_NONE), so this function should also check
that the initial vertex that is going to be used to find the boundary is
a valid index.

Reviewed By: sergey

Maniphest Tasks: T79754

Differential Revision: https://developer.blender.org/D8860