Fix use-after-release issue with pg_identify_object_as_address()

Spotted by buildfarm member prion, with -DRELCACHE_FORCE_RELEASE.

Introduced in f7aab36d.

Discussion: https://postgr.es/m/2759018.1619577848@sss.pgh.pa.us
Backpatch-through: 9.6

src/backend/catalog/objectaddress.c

@@ -5619,7 +5619,7 @@ getObjectIdentityParts(const ObjectAddress *object,
 					break;
 				}
 				trigForm = (Form_pg_event_trigger) GETSTRUCT(tup);
-				evtname = NameStr(trigForm->evtname);
+				evtname = pstrdup(NameStr(trigForm->evtname));
 				appendStringInfoString(&buffer, quote_identifier(evtname));
 				if (objname)
 					*objname = list_make1(evtname);