rustls_error: correct output buffer length handling

This function does not need to keep an output buffer byte
spare to zero-terminate the string, as it doesn't terminate
the output.  So don't do that.

As an effect, this fixes the integer underflow followed by
buffer overflow for `rustls_error` with an empty output buffer.
`test_rustls_error_into_empty_buffer` is a regression test for
that case.

Author: ctz

librustls/src/error.rs

@@ -235,7 +235,7 @@ impl rustls_result {
                 return;
             }
             let error_str = rustls_result::from(result).to_string();
-            let out_len = min(len - 1, error_str.len());
+            let out_len = min(len, error_str.len());
             unsafe {
                 std::ptr::copy_nonoverlapping(error_str.as_ptr() as *mut c_char, buf, out_len);
                 *out_n = out_len;
@@ -785,6 +785,13 @@ mod tests {
         assert_eq!(&output, "peer sent no certificates");
     }
 
+    #[test]
+    fn test_rustls_error_into_empty_buffer() {
+        let mut n = 99;
+        rustls_result::rustls_error(0, &mut [] as *mut _, 0, &mut n);
+        assert_eq!(n, 0);
+    }
+
     #[test]
     fn test_rustls_result_is_cert_error() {
         assert!(!rustls_result::rustls_result_is_cert_error(0));