Fix rotation validity test in image.c

* src/image.c (compute_image_rotation): Fix the validity test
for :rotation values.  This avoids logging error messages when
no :rotation was provided in the image spec.
1 job for master in 56 minutes and 10 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #2359


Name Stage Failure
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
make: *** [check-expensive] Error 2
Makefile:959: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1