Merge #130: Fix index out of bounds bug

ea644f0 decode: Add unit test for parsing segwit address (Tobin C. Harding)
5d6e5c3 decode: Add empty data check (Tobin C. Harding)

Pull request description:

  Noob bug here, I accessed an array without first checking that it was non-empty :(

  Adds unit test as a separate patch, re-arrange to verify the bug.

  Discovered while fuzzing #128.

ACKs for top commit:
  apoelstra:
    ACK ea644f0

Tree-SHA512: dda0b91f8e2f94ba47b0d6f386e1edc17938bf4c71851ec387f922389ba384693655ed863e98bb95f60970bbf36844cc6c58b654c666b6ab1116cd6dea970ba2

Author: apoelstra

src/primitives/decode.rs

@@ -368,6 +368,10 @@ impl<'s> SegwitHrpstring<'s> {
     pub fn new(s: &'s str) -> Result<Self, SegwitHrpstringError> {
         let unchecked = UncheckedHrpstring::new(s)?;
 
+        if unchecked.data.is_empty() {
+            return Err(SegwitHrpstringError::MissingWitnessVersion);
+        }
+
         // Unwrap ok since check_characters (in `Self::new`) checked the bech32-ness of this char.
         let witness_version = Fe32::from_char(unchecked.data[0].into()).unwrap();
         if witness_version.to_u8() > 16 {
@@ -943,4 +947,26 @@ mod tests {
             assert!(CheckedHrpstring::new::<Bech32>(valid).is_ok())
         }
     }
+
+    macro_rules! check_invalid_segwit_addresses {
+        ($($test_name:ident, $reason:literal, $address:literal);* $(;)?) => {
+            $(
+                #[test]
+                fn $test_name() {
+                    let res = SegwitHrpstring::new($address);
+                    if res.is_ok() {
+                        panic!("{} sting should not be valid: {}", $address, $reason);
+                    }
+                }
+            )*
+        }
+    }
+    check_invalid_segwit_addresses! {
+        invalid_segwit_address_0, "missing hrp", "1qar0srrr7xfkvy5l643lydnw9re59gtzzwf5mdq";
+        invalid_segwit_address_1, "missing data-checksum", "91111";
+        invalid_segwit_address_2, "invalid witness version", "bc14r0srrr7xfkvy5l643lydnw9re59gtzzwf5mdq";
+        invalid_segwit_address_3, "invalid checksum length", "bc1q5mdq";
+        invalid_segwit_address_4, "missing data", "bc1qwf5mdq";
+        invalid_segwit_address_5, "invalid program length", "bc14r0srrr7xfkvy5l643lydnw9rewf5mdq";
+    }
 }