dpif-linux: Avoid null dereference if all ports disappear.

When dpif_linux_refresh_channels() refreshes the set of channels when
the number of handlers changes, it destroys all the dpif's channels and
sets dpif->uc_array_size to 0.  If the port dump later in the function
turns up no ports (which generally indicates a bug), then no channels will
be allocated and thus dpif->uc_array_size will remain 0 and 'channels' will
be null in each handler.  This is self-consistent, at least, but
dpif_linux_port_get_pid__() was still willing in this situation to
try to access element 0 of the set of channels, dereferencing a null
pointer.

This fixes the problem.

I encountered this while looking at a bug that I had introduced during
development that caused the port dump to always be empty.  It would be
difficult to encounter in normal use.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>