Fix NULL pointer dereference in zdb

Clang's static analyzer complained that we dereference a NULL pointer in
dump_path() if we return 0 when there is an error.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes openzfs#14044

Author: ryao

cmd/zdb/zdb.c

@@ -2995,7 +2995,7 @@ open_objset(const char *path, void *tag, objset_t **osp)
 	}
 	sa_os = *osp;
 
-	return (0);
+	return (err);
 }
 
 static void