Fixed memory leak due to incorrect freeing of DTLS reassembly bit mask

PR#3608

(Imported from upstream's 8a35dbb6d89a16d792b79b157b3e89443639ec94.)

Change-Id: Iab9d91f9b96793f2275a23770f1275ff4edf0386
Reviewed-on: https://boringssl-review.googlesource.com/2476
Reviewed-by: Adam Langley <agl@google.com>