cursor: fix crash with weird input files

If a cursor file contains multiple images for the same size, this
typically indicates an animation. The compositor weston uses
wl_cursor_frame_and_duration to figure out at which time a specific image
should be shown.

The total delay is the sum of all image delays. But if all images have a
delay of 0, the total delay is 0 as well. The code does not check for this
special condition and triggers a floating point exception by eventually
performing a modulo operation with 0.

This, of course, could also happen if the sum of all image delays
triggers an unsigned int overflow. But since a comment in the code
already indicates that it does not try to "fix" handling of weird files,
I would argue that it's "okay" if that happens. At least the program
won't crash.

Proof of Concept:

install -D ~/.icons/poc/cursors
base64 -d > ~/.icons/poc/cursors/left_ptr << EOF
WGN1chAAAAAAAAEAAgAAAAIA/f8BAAAAKAAAAAIA/f8BAAAAKAAAACQAAAACAP3/AQAAAAEAAAAB
AAAAAQAAAAEAAAABAAAAAAAAAAAAAAA=
EOF
cat > /tmp/weston.ini << EOF
[shell]
cursor-theme=poc
EOF
weston -c /tmp/weston.ini

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>