pg_waldump: Fix small memory leak when rmgr->rm_identify returns NULL.

This got broken in 604f7956, shortly after rm_identify's
introduction.

Author: Andres Freund
Discussion: https://postgr.es/m/20191029233341.4gnyau7e5v2lh5sc@alap3.anarazel.de
Backpatch: 9.5, where rm_identify was introduced