Fix empty fragment handling in DTLS message reassembly.
Found with libFuzzer.
Bug: chromium:763097
Change-Id: I806bcfc714c0629ff7f725e37f4c0045d4ec7ac6
Reviewed-on: https://boringssl-review.googlesource.com/20105
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Steven Valdez <svaldez@google.com>
Reviewed-by: Steven Valdez <svaldez@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>