[ProBuilder] Merging/welding vertices can result in out of bounds vertex references and exceptions

In certain cases, welding/merging vertices on a ProBuilder shape can produce out of bound vertex references. This is most easily seen when merging 7/8 vertices of a ProBuilder cube, resulting in a series of errors/exceptions as follows (also see attached text with callstack preserved):

Failed setting triangles. Some indices are referencing out of bounds vertices. IndexCount: 2, VertexCount: 0
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

KeyNotFoundException: The given key was not present in the dictionary.
System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) (at <a8ed250850854b439cedc18931a314fe>:0)

KeyNotFoundException: The given key was not present in the dictionary.
System.Collections.Generic.Dictionary`2[TKey,TValue].get_Item (TKey key) (at <a8ed250850854b439cedc18931a314fe>:0)

GUI Error: You are pushing more GUIClips than you are popping. Make sure they are balanced.
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)

Steps to reproduce:
- Create a new Unity 2019.3 3D project and install ProBuilder (or open attached project)
- In any scene, create a new ProBuilder Cube
- In vertex editing mode, select 7 of the 8 vertices
- Use the "Weld Vertices" tool
- Observe errors/excepts regarding out of bounds vertices