test suite crashes in nearestscale

[rtandy]

The test suite crashes at scale.c:175 in nearestscale. The reason seems to be the way nearestscale is invoked:

        nearestscale(&img, 1280, 1024, 1280, to);

In nearestscale, width and bytesperline are the same which causes jdy to be negative -- or rather, since it's unsigned, positive and very large. At the end of the first outer loop it gets added to newBuf, causing a segfault on the next access to newBuf at line 175.

[andreasbaumann]

vagrind says:

==56796== 1 errors in context 1 of 1:
==56796== Invalid write of size 1
==56796==    at 0x10D096: nearestscale (scale.c:175)
==56796==    by 0x10A760: test_nearestscale (test.c:40)
==56796==    by 0x10A89F: main (test.c:59)
==56796==  Address 0x105206080 is not stack'd, malloc'd or (recently) free'd