Fix psbt fuzz crash

Fixes: https://github.com/rust-bitcoin/rust-bitcoin/issues/3628

This occurs when combining two PSBTs with different xpub key sources. Added a length check before indexing into slices to prevent out-of-bounds access.

bitcoin/src/psbt/mod.rs

@@ -255,8 +255,9 @@ impl Psbt {
                                 == derivation2[derivation2.len() - derivation1.len()..])
                     {
                         continue;
-                    } else if derivation2[..]
-                        == derivation1[derivation1.len() - derivation2.len()..]
+                    } else if derivation2.len() <= derivation1.len()
+                        && derivation2[..]
+                                == derivation1[derivation1.len() - derivation2.len()..]
                     {
                         entry.insert((fingerprint1, derivation1));
                         continue;
@@ -2113,6 +2114,16 @@ mod tests {
         assert_eq!(psbt1, psbt2);
     }
 
+
+    // https://github.com/rust-bitcoin/rust-bitcoin/issues/3628
+    #[test]
+    fn test_combine_psbt_fuzz_3628() {
+        let mut psbt1 = hex_psbt(include_str!("../../tests/data/psbt_fuzz1.hex")).unwrap();
+        let psbt2 = hex_psbt(include_str!("../../tests/data/psbt_fuzz2.hex")).unwrap();
+
+        assert!(matches!(psbt1.combine(psbt2).unwrap_err(), Error::CombineInconsistentKeySources(_)));
+    }
+
     #[cfg(feature = "rand-std")]
     fn gen_keys() -> (PrivateKey, PublicKey, Secp256k1<All>) {
         use secp256k1::rand::thread_rng;

bitcoin/tests/data/psbt_fuzz1.hex

@@ -0,0 +1 @@
+70736274ff01000a000000ff0000000074ff4f010488b21eff02000001004a92244992244902030203030303030303030303030303030303030303030303030303030303030303f4000000000000000a000208ffffffff08080804000000000000000c080808000b0000000000010000

bitcoin/tests/data/psbt_fuzz2.hex

@@ -0,0 +1 @@
+70736274ff01000a000000ff0000000074ff4f010488b21eff02000001004a92244992244902030203030303030303030303030303030303030303030303030303030303030303f4000000000000000a000208ffffffff080808040000000000000008000000000000001000