Reject too-long IPs quicker

Now that there can't be a bunch of leading zeros, parsing can be
optimized a bit.

library/std/src/net/parser.rs

@@ -289,7 +289,12 @@ impl FromStr for IpAddr {
 impl FromStr for Ipv4Addr {
     type Err = AddrParseError;
     fn from_str(s: &str) -> Result<Ipv4Addr, AddrParseError> {
-        Parser::new(s).parse_with(|p| p.read_ipv4_addr())
+        // don't try to parse if too long
+        if s.len() > 15 {
+            Err(AddrParseError(()))
+        } else {
+            Parser::new(s).parse_with(|p| p.read_ipv4_addr())
+        }
     }
 }