Improve test coverage of ScriptBuf

diff --git a/primitives/src/script/owned.rs b/primitives/src/script/owned.rs
index ffe68bc5..207e1c1a 100644
--- a/primitives/src/script/owned.rs
+++ b/primitives/src/script/owned.rs
@@ -282,6 +282,12 @@ mod tests {
         assert!(script.capacity() >= 10);
     }
 
+    #[test]
+    fn script_buf_default() {
+        let script: ScriptBuf = ScriptBuf::default();
+        assert!(script.is_empty());
+    }
+
     #[test]
     fn script_consensus_decode_empty() {
         let bytes = vec![0_u8];
@@ -308,4 +314,22 @@ mod tests {
 
         assert_eq!(got, want);
     }
+
+    #[test]
+    fn decoder_full_read_limit() {
+        let mut decoder = ScriptBuf::decoder();
+        // ByteVecDecoder length prefix is CompactSize: needs 1 byte.
+        assert_eq!(decoder.read_limit(), 1);
+
+        // Script length prefix = 32.
+        let mut push = [32_u8].as_slice();
+        decoder.push_bytes(&mut push).unwrap();
+        // Limit is 32 for the script data.
+        assert_eq!(decoder.read_limit(), 32);
+
+        // Provide 1 byte of script data decreasing the read limit by 1.
+        let mut push = [0xAA_u8].as_slice();
+        decoder.push_bytes(&mut push).unwrap();
+        assert_eq!(decoder.read_limit(), 31);
+    }
 }