camel: Avoid an unlikely division by zero on a race condition
camel: Avoid an unlikely division by zero on a race condition

This is a subtle and really unlikely TOCTTOU race which Coverity found.
Fix the potential division by zero which results from it.

This fixes another instance of the original problem, fixed in commit
8fb4543.

Coverity issue: #1061271

https://bugzilla.gnome.org/show_bug.cgi?id=730376

tintou/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server,tintou/evolution-data-server,matzipan/evolution-data-server