rv34: check that subsequent slices have the same type as first one.

rv34: check that subsequent slices have the same type as first one.

This prevents some crashes when corrupted bitstream reports e.g. P-type
slice in I-frame. Official RealVideo decoder demands all slices to be
of the same type too.

Signed-off-by: Anton Khirnov <anton@khirnov.net>