Fix use of relcache TriggerDesc field introduced by commit
05c8482f7f.
The commit added code which used a relcache TriggerDesc field across
another cache access, which it shouldn't because the relcache doesn't
guarantee it won't get moved.
Diagnosed-by: Tom Lane
Author: Greg Nancarrow
Reviewed-by: Hou Zhijie, Amit Kapila
Discussion: https://postgr.es/m/
2309260.
1615485644@sss.pgh.pa.us