ofp-parse: Fix GCC compiler warnings with -O3.

When compiling with -O3, GCC can no longer tell that the following
warnings are not valid.  At any rate, they're easy enough to silence.

ofp-parse.c:764:45: error: 'ttl' may be used uninitialized in this
function

ofp-parse.c:752:17: error: 'ecn' may be used uninitialized in this
function

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