a thread count of 0 is treated the same as 1, use av_cpu_count() to get
the correct thread count when auto threads is requested.

this matches the fix in libvpxenc:
27df34bf avcodec/libvpxenc: fix setting amount of threads used for encoding
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: James Zern <jzern@google.com>