Prevent http://[] from causing panic

The code:

```
Url::parse("http://[\n:]")
```

The panic:

```
thread '<main>' panicked at 'index out of bounds: the len is 0 but the index is 0',
/Users/coreyf/.cargo/registry/src/github.com-1ecc6299db9ec823/url-0.2.34/src/host.rs:104
```

Was found using https://github.com/kmcallister/afl.rs 👍

src/host.rs

@@ -101,6 +101,11 @@ impl Ipv6Address {
         let mut piece_pointer = 0;
         let mut compress_pointer = None;
         let mut i = 0;
+
+        if len == 0 {
+            return Err(ParseError::InvalidIpv6Address)
+        }
+
         if input[0] == b':' {
             if input[1] != b':' {
                 return Err(ParseError::InvalidIpv6Address)

src/urltestdata.txt

@@ -267,6 +267,9 @@ http://Goo%20\sgoo%7C|.com
 # This should fail
 http://GOO\u00a0\u3000goo.com
 
+# This should fail
+http://[]
+
 # Other types of space (no-break, zero-width, zero-width-no-break) are
 # name-prepped away to nothing.
 XFAIL http://GOO\u200b\u2060\ufeffgoo.com  s:http p:/ h:googoo.com