Work around an apparent binding bug (GCC 4.8).

A reference to h->seq passed to std::pair ostensibly could not bind
because the header structure is packed.  At first this looked like
a more general unaligned access problem, but the only location the
compiler rejects is a false positive.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
(cherry picked from commit c930a1f119069a424af28a618b0abff4947c221f)