num2words: catch NAN case

NAN string is parsed by the BigFloat package successfully (as it is when
parsed into any number type in Rust). This means the creation of the
Num2Words object succeeds and tries to parse that number. However, this
cause the call .to_u64().unwrap() to crash with the following error:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /home/asp/.cargo/registry/src/index.crates.io-6f17d22bba15001f/num2words-1.0.0/src/lang/en.rs:78:53

Closes #12

src/num2words.rs

@@ -145,6 +145,9 @@ impl Num2Words {
     /// ```
     pub fn parse(num: &str) -> Option<Self> {
         let num = BigFloat::parse(num)?;
+        if num.is_nan() {
+            return None;
+        }
         Some(Self {
             num,
             lang: Lang::English,
@@ -320,5 +323,9 @@ mod tests {
             Some(_) => assert!(false),
             None => assert!(true),
         }
+        match Num2Words::parse("NAN") {
+            Some(_) => assert!(false),
+            None => assert!(true),
+        }
     }
 }