Fix 0012471: Bug formatted IDs not escaped properly for DOT relgraph

Bug IDs can be pretty formatted by plugins handling the
EVENT_DISPLAY_BUG_ID event. The returned pretty formatted ID may not be
safe to dump directly into the DOT language script used to generate
relationship graphs. Therefore we need to check for a valid format
according to DOT language rules. If the format returned is invalid then
we use the raw bug ID (non-padded) instead.

Thanks to Jimmy Monin for reporting this issue and providing assistance.