Prevent use-after-free with ctx->ns in set_nameservers(..), which could occur

if the memory wasn't allocated again later on

Reported by: Coverity
Submitted by: Miles Ohlrich <miles.ohlrich@isilon.com>
MFC after: 1 week
Sponsored by: EMC / Isilon Storage Division

usr.sbin/uhsoctl/uhsoctl.c

@@ -452,6 +452,7 @@ set_nameservers(struct ctx *ctx, const char *respath, int ns, ...)
 			free(ctx->ns[i]);
 		}
 		free(ctx->ns);
+		ctx->ns = NULL;
 	}
 
 	fd = open(respath, O_RDWR | O_CREAT | O_NOFOLLOW, 0666);