Clear *out in ReadHandshakeData's empty case.

This is test-only code and a no-op, but it's prudent to avoid making
assumptions about the initial state of *out. Hopefully someday later we can
assume std::optional or pull in absl::optional.

Change-Id: I85af87bb2cc3cda3d40801c91e6abe4f5a7d89f8
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/37184
Commit-Queue: David Benjamin <davidben@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
diff --git a/ssl/ssl_test.cc b/ssl/ssl_test.cc
index 4b92acd..2df005a 100644
--- a/ssl/ssl_test.cc
+++ b/ssl/ssl_test.cc
@@ -4810,6 +4810,7 @@
     }
     // The peer may not have configured any keys yet.
     if (peer_->levels_[level].write_secret.empty()) {
+      out->clear();
       return true;
     }
     // Check the peer computed the same key.