Don't crash when we don't know the specified font

This was reported by internal testing by @aterrong and has been found in crashreport.zentyal.org id: 2058

#### Issue

Samba crash when outlook client is syncing data, detected with appointments. Only _sometimes_ I've reproduced it twice in a row and then no more :pensive:.

#### Cause

Segfault caused for null pointer on rtf parser trying to obtain a font that we don't know. Actually it wasn't in the font table of the rtf document (bad formed?).

#### Solution

Ignore it :see_no_evil: