bug#21271: 25.0.5; `field' text-property screws with `line-end-position'

[Artur Malabarba]

1. In an empty buffer, hit RET and type some text on the second line.
2. Place point at the start of the second line.
3. Run (line-end-position) and verify that the value is correct
(something greater than 3)
4. Run (add-text-properties (1- (point)) (point) '(field 1))
5. Run (line-end-position) and you'll get 2 (which is the poistion of
the end of the field).