test-classifier: add ovs_assert to prevent warning

GCC 4.9.0 triggers a warning (array-bounds) while compiling test-classifier.c
This commit introduces an assertion that suppresses the warning.

Signed-off-by: Daniele Di Proietto <ddiproietto@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>