iproute2: reject zero length strings for matches

The ip commands use the function matches() to allow for abbreviations like:

  $ ip l
  $ ip r

But the function does not check for zero length strings which is potentially error prone (but might also break some power users assumptions). For example:

$ ip ""
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
       valid_lft forever preferred_lft forever

This patch checks for zero length strings and never matches the option.

$ ip ""
Object "" is unknown, try "ip help".

index ac155bf5a044..b68a2e0f4a07 100644
@@ -733,7 +733,7 @@ int matches(const char *cmd, const char *pattern)
 {
 	int len = strlen(cmd);
 
-	if (len > strlen(pattern))
+	if (len == 0 || len > strlen(pattern))
 		return -1;
 	return memcmp(pattern, cmd, len);
 }