Prevent drawing an extra line in draw_line_segment_mut
Prevent drawing an extra line in draw_line_segment_mut

In cases where `is_steep` was `true`, (y, x) was out of bounds, but
(x, y) was in bounds, the `else` clause would draw an extra line segment
by mirroring the out-of-bounds part of the line into the image.

PistonDevelopers/imageproc,theotherphil/imageproc