avformat: prevent triggering request_probe assert in ff_read_packet

If probe_codec is called with pkt == NULL, it sets probe_packets to 0
and request_probe to -1.
However, request_probe can change when calling s->iformat->read_packet
and thus a probe_packets value of 0 doesn't guarantee a request_probe
value of -1.
In that case calling probe_codec again is necessary to prevent
triggering the assert.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>
