ofp-print: Fix misaligned data access in ofp_print_error_msg().

The body of an OpenFlow error message often contains an inner OpenFlow
message, and when it does, the inner message starts at an odd multiple of 4
bytes from the beginning of the outer message.  That means that, on RISC
systems, accessing the inner message directly causes a bus error.  This
commit fixes the problem in a way that should make it difficult to recur.

This fixes the failure of tests 643, 645, and 651 on sparc seen here:
https://buildd.debian.org/status/fetch.php?pkg=openvswitch&arch=sparc&ver=2.1.0%2Bgit20140325-1&stamp=1396438624

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>