Try working out how deep the threaded portion of the insert in the back of the TV is (use a paper-clip or similar - straighten it out then insert into the threaded hole and slide up the side wall of the hole - you should be able to feel the depth of the threaded insert)
Any length of the bolt that protrudes further than the threaded portion will be in fresh air and therefore won't be providing any additional support. Just use a fine tooth hacksaw to saw the bolt to length then a file to smooth off any burrs.
In truth, in any nut and bolt assembly it is only the first couple of threads that actually provide strength / clamp force to the joint, due to the clearances built into the design of the two elements and minute differences in the pitch of both threads.
Hope the above helps and didn't sound condescending
Oh, and be careful with that paperclip!!