Fix CVE-2014-3507, avoid allocating and then leaking a fresh fragment
structure when a zero-length fragment is received.

Based on
https://git.openssl.org/gitweb/?p=openssl.git;a=commitdiff;h=d0a4b7d1a2948fce38515b8d862f43e7ba0ebf74

diff by miod@, ok guenther@ bcook@ deraadt@