Fix some memory leaks associated with parsing json and manifests

Coverity complained about not freeing some memory associated with
incrementally parsing backup manifests. To fix that, provide and use a new
shutdown function for the JsonManifestParseIncrementalState object, in
line with a suggestion from Tom Lane.

While analysing the problem, I noticed a buglet in freeing memory for
incremental json lexers. To fix that remove a bogus condition on
freeing the memory allocated for them.

src/backend/backup/basebackup_incremental.c

@@ -241,6 +241,9 @@ FinalizeIncrementalManifest(IncrementalBackupInfo *ib)
 	pfree(ib->buf.data);
 	ib->buf.data = NULL;
 
+	/* Done with inc_state, so release that memory too */
+	json_parse_manifest_incremental_shutdown(ib->inc_state);
+
 	/* Switch back to previous memory context. */
 	MemoryContextSwitchTo(oldcontext);
 }