Merge rust-bitcoin#2837: pow: Fix off-by-one error

dd80a08 pow: Unit test from_hex_internal (Tobin C. Harding)
c16f7ad pow: Fix off-by-one error (Tobin C. Harding)

Pull request description:

  This is a backport of the 2 patches in rust-bitcoin#2834, grammar mistakes in the commit log and all.

  Note, one difference, we call `hex_u128` instead of `hex_u128_unchecked` because the unchecked version does not exist in `v0.32`.

ACKs for top commit:
  apoelstra:
    ACK dd80a08
  Kixunil:
    ACK dd80a08

Tree-SHA512: c93d47fb57105523284600e81f695091afb9d1c6b21dc4481b81c97e5fe97fc3842ae15c563f266166d18eff14bf256e550604ab2999f2b878393ae6a0ca8bdb

Author: apoelstra

bitcoin/src/pow.rs

@@ -434,7 +434,7 @@ impl U256 {
 
     // Caller to ensure `s` does not contain a prefix.
     fn from_hex_internal(s: &str) -> Result<Self, ParseIntError> {
-        let (high, low) = if s.len() < 32 {
+        let (high, low) = if s.len() <= 32 {
             let low = parse::hex_u128(s)?;
             (0, low)
         } else {
@@ -1589,6 +1589,14 @@ mod tests {
         assert_eq!(got, val);
     }
 
+    #[test]
+    fn u256_from_hex_32_characters_long() {
+        let hex = "a69b455cd41bb662a69b4555deadbeef";
+        let want = U256(0x00, 0xA69B_455C_D41B_B662_A69B_4555_DEAD_BEEF);
+        let got = U256::from_unprefixed_hex(hex).expect("failed to parse hex");
+        assert_eq!(got, want);
+    }
+
     #[cfg(feature = "serde")]
     #[test]
     fn u256_serde() {