test for panic if float is too large for real8

src/basic.rs

@@ -275,10 +275,16 @@ mod tests {
 
         assert_eq!(decode_real8(0x4110 << 48), 1.0);
         assert_eq!(decode_real8(0xC120 << 48), -2.0);
-
-        //TODO panics on invalid?
     }
 
+    #[test]
+    #[should_panic]
+    fn test_encode_real8_panic() {
+        use crate::basic::encode_real8;
+        encode_real8(1e80);
+    }
+
+
     #[test]
     fn test_parse_real8() {
         use crate::basic:: parse_real8;