tests: loaders: Add jpeg CMYK loader test.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: JPG: Fix segfault in load_cmyk()

Fixes typo in number of scanlines to load from a CMYK jpeg.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>