ofproto-dpif: Avoid potential undefined behavior in type_run().

ofproto-dpif: Avoid potential undefined behavior in type_run().

When HMAP_FOR_EACH completes, the value in the loop control variable is not
necessarily NULL.  It is NULL minus the offset of the hmap_node struct
member, which is nonnull if that offset is nonzero.  Currently,
'all_ofproto_dpifs_node' is the first member in struct ofproto_dpif, so
there is no real bug, but there would be if the struct were rearranged.

This commit heads off the problem by avoiding any assumption about the
loop control variable after HMAP_FOR_EACH.

Signed-off-by: Ben Pfaff <blp@nicira.com>