This commit fixes [[ https://developer.blender.org/T79590 | T79590 UV Island Won't Unwrap If Only One Vertex Is Pinned ]]
When unwrapping UV island with only one pinned UV, it will find 2 extreme vertices along the primal axis, and pin whichever is farther from the pinned vertex.
1. Pin only one vertex
{F8752432}
2. Select all and unwrap
{F8752437}