Add length check for Day 4, hcl value

src/day4.rs

@@ -76,6 +76,9 @@ impl Creds {
     }
 
     fn validate_hcl(val: &str) -> bool {
+        if val.len() != 7 {
+            return false;
+        }
         let mut ch = val.chars();
         if ch.next() != Some('#') {
             return false;