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

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

This got broken in 604f7956b94, 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

Branch
------
REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/95f2efd5041b95cb92e3940e79cbbb65c4c4b09e

Modified Files
--------------
src/bin/pg_xlogdump/pg_xlogdump.c | 11 ++++++-----
1 file changed, 6 insertions(+), 5 deletions(-)