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 #21068

core/collapse_api.php

@@ -224,7 +224,7 @@ function collapse_cache_token() {
 		if( $t_update ) {
 			$t_value = json_encode( $g_collapse_cache_token );
 			token_set( TOKEN_COLLAPSE, $t_value, TOKEN_EXPIRY_COLLAPSE );
-		} else {
+		} elseif( token_exists( $t_token['id'] ) ) {
 			token_touch( $t_token['id'] );
 		}