resource_exists in trac.ticket.api should avoid database exceptions from invalid input

[Ryan J Ollos]

Reported in ​th:#12253, the function resource_exists in trac.ticket.api raises an exception on PostgreSQL when resource.id is not an integer.

Perhaps we should simply try converting resource.id to an integer resource_exists​:

try:
   id_ = int(resource.id)
except ValueError:
   return False