MEDIUM: sample: Small fix in function check_operator for eror reporting

When function "check_operator" calls function "vars_check_arg" to decode
a variable, it passes in NULL value for pointer to the char array meant
for capturing the error message.  This commit replaces NULL with the
pointer to the real char array.  This should help in correct error
reporting.

src/sample.c

@@ -3156,7 +3156,7 @@ static int check_operator(struct arg *args, struct sample_conv *conv,
 	long long int i;
 
 	/* Try to decode a variable. */
-	if (vars_check_arg(&args[0], NULL))
+	if (vars_check_arg(&args[0], err))
 		return 1;
 
 	/* Try to convert an integer */