Make the code setting uv_angle match the AV1 spec

The following snippet is found in the AV1 spec in intra_frame_mode_info( ) and intra_block_mode_info( ):

        if ( UVMode == UV_CFL_PRED ) {
            read_cfl_alphas( )
        }
        intra_angle_info_uv( )

Note that there is no 'else' after the if statement. Therefore, in decode_b(), the code that sets b->uv_angle should not be preceded by an 'else'.