Fix error handling in hex_decode.

Problem found while refactoring usage in examples/turnserver/.

Bug: webrtc:6424
Change-Id: Ib1d54055c5914136b5bf165d48ab7d19520ff967
Reviewed-on: https://webrtc-review.googlesource.com/c/108302
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#25412}
diff --git a/rtc_base/stringencode_unittest.cc b/rtc_base/stringencode_unittest.cc
index 9bdc592..f21c4cb 100644
--- a/rtc_base/stringencode_unittest.cc
+++ b/rtc_base/stringencode_unittest.cc
@@ -141,7 +141,8 @@
 
 // Test that decoding non-hex data fails.
 TEST_F(HexEncodeTest, TestDecodeBogusData) {
-  dec_res_ = hex_decode_with_delimiter(decoded_, sizeof(decoded_), "xyz", 3, 0);
+  dec_res_ =
+      hex_decode_with_delimiter(decoded_, sizeof(decoded_), "axyz", 4, 0);
   ASSERT_EQ(0U, dec_res_);
 }