fix according to the comments

src/runner.rs

@@ -1,6 +1,5 @@
 use std::ops;
 use std::{u32, usize};
-use std::cmp;
 use std::fmt;
 use std::iter::repeat;
 use parity_wasm::elements::Local;
@@ -1262,13 +1261,15 @@ impl ValueStack {
 
 	#[inline]
 	fn extend(&mut self, values: &[RuntimeValue]) -> Result<(), TrapKind> {
-		let len = cmp::min(values.len(), self.buf.len() - self.sp);
-		self.buf[self.sp..(self.sp+len)].copy_from_slice(&values[..len]);
-		self.sp += len;
-		if len < values.len() {
-			Err(TrapKind::StackOverflow)
+		let avail_len = self.buf.len() - self.sp;
+		let to_copy_len = values.len();
+
+		if to_copy_len <= avail_len {
+		    self.buf[self.sp..(self.sp+to_copy_len)].copy_from_slice(&values[..to_copy_len]);
+		    self.sp += to_copy_len;
+		    Ok(())
 		} else {
-			Ok(())
+		    Err(TrapKind::StackOverflow)
 		}
 	}
 
@@ -1286,8 +1287,13 @@ mod tests {
 	#[test]
 	fn test_stack_extend() {
 		let mut stack = ValueStack::with_limit(5);
-		let values = vec![RuntimeValue::I32(1), RuntimeValue::I32(2), RuntimeValue::I32(3),
-			RuntimeValue::I32(4), RuntimeValue::I32(5)];
+		let values = vec![
+			RuntimeValue::I32(1),
+			RuntimeValue::I32(2),
+			RuntimeValue::I32(3),
+			RuntimeValue::I32(4),
+			RuntimeValue::I32(5)
+		];
 
 		assert!(stack.extend(&values).is_ok());
 		assert_eq!(stack.len(), 5);
@@ -1303,4 +1309,28 @@ mod tests {
 
 		assert_eq!(stack.len(), 0);
 	}
+
+	#[test]
+	fn test_stack_extend_overflow() {
+		let mut stack = ValueStack::with_limit(4);
+		let values = vec![
+			RuntimeValue::I32(1),
+			RuntimeValue::I32(2),
+			RuntimeValue::I32(3),
+			RuntimeValue::I32(4),
+			RuntimeValue::I32(5)
+		];
+
+		assert!(stack.extend(&values).is_err());
+		assert_eq!(stack.len(), 0);
+
+		assert!(stack.push(RuntimeValue::I32(6)).is_ok());
+
+		assert_eq!(stack.len(), 1);
+
+		// check each elements
+		assert_eq!(stack.pop(), RuntimeValue::I32(6));
+
+		assert_eq!(stack.len(), 0);
+	}
 }