Fix intermittent "error 2300 token not found"

In some rare cases, collapse_cache_token() would attempt to touch a
token that does not exist. This can happen when the current user does
not have any TOKEN_COLLAPSE token, and the MANTIS_collapse_cookie
contains a non-empty value that does not form a valid, colon-delimited
pair.

The issue has been addressed by adding a token_exists() check prior to
calling token_touch().

Fixes 0021068