psbt: return ErrInvalidKeydata if value isn't a 32-bit uint

This commit fixes a panic when deserializing PSBTs in raw binary.
If the key type was SighashType and the value was not 4 bytes long,
the call to binary.LittleEndian.Uint32(value) would panic as the
function expects 4 bytes to parse into a uint32. We now perform a
sanity check that asserts that the value is 4 bytes long.

psbt/partial_input.go

@@ -141,6 +141,12 @@ func (pi *PInput) deserialize(r io.Reader) error {
 				return ErrInvalidKeydata
 			}
 
+			// Bounds check on value here since the sighash type must be a
+			// 32-bit unsigned integer.
+			if len(value) != 4 {
+				return ErrInvalidKeydata
+			}
+
 			shtype := txscript.SigHashType(
 				binary.LittleEndian.Uint32(value),
 			)