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>

ssl/d1_both.cc

diff --git a/ssl/d1_both.cc b/ssl/d1_both.cc
index 321e01d..ab499df 100644
--- a/ssl/d1_both.cc
+++ b/ssl/d1_both.cc
@@ -228,6 +228,10 @@
   // A zero-length message will never have a pending reassembly.
   assert(msg_len > 0);
 
+  if (start == end) {
+    return;
+  }
+
   if ((start >> 3) == (end >> 3)) {
     frag->reassembly[start >> 3] |= bit_range(start & 7, end & 7);
   } else {