vty: Fix left shifting out of range on signed variable

Fixes following ASan runtime errors while running vty tests:
command.c:730:27: runtime error: left shift of 1 by 31 places cannot be represented in type 'int'

Somehow we didn't catch this one in Ie11ff18d6fd9f6e1e91a51b6156fb6b0b7d3a9a8

Change-Id: I601caf7daa947f3cf391316f1011007ef9188c90
diff --git a/src/vty/command.c b/src/vty/command.c
index b38be34..4825109 100644
--- a/src/vty/command.c
+++ b/src/vty/command.c
@@ -727,7 +727,7 @@
 			char flag;
 
 			/* Skip attribute if *not* set */
-			if (~cmd->usrattr & (1 << i))
+			if (~cmd->usrattr & ((unsigned)1 << i))
 				continue;
 
 			xml_att_desc = xml_escape(desc[i]);