proresdec_lgpl: explicitly check coff[3] against slice_data_size

The implicit checks via v_data_size and a_data_size don't work in the case
'(hdr_size > 7) && !ctx->alpha_info'.

This fixes segmentation faults due to invalid reads.

This problem was introduced in commit
547c2f002a87f4412a83c23b0d60364be5e7ce58.

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