fix: fall-through to default cursor in cursor_get_from_edge()

fix: fall-through to default cursor in cursor_get_from_edge()

When assertions are disabled, providing an unexpected input to
cursor_get_from_edge will cause the non-void function to terminate
without a return value. This may be effectively unreachable in practice.
However, returning a default cursor as a fall-through case will both
silence a compiler warning and prevent catastrophy should the function
ever be called with a permitted value.