ofproto-dpif-upcall: Properly initialize 'recv_buf'.

Due to a typo, the latest upcall refactoring caused dpif_recv() to be
called on an un-initialized chunk of memory.

Signed-off-by: Ethan Jackson <ethan@nicira.com>
Reported-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>