Fix crash in finding path over navmesh (#6338)

elsidrequested to merge
elsid/openmw:fix_find_path_crash into master
  • Avoid access to the path vector element out of range. polygonPath.front() in some cases might reference to a first element of empty vector. Copy the value into a local variable to be able to access later.
  • Avoid redundant polygon path reallocations. Use separate variable to store size and make all operations in-place.

Better to review each change in a separate commit.

Fixes #6338 (closed).

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