mvcdec: Heap overflow in 'ih264d_parse_fgc'

Although the fag end of both the NALU and the bitstream buffer
 is being parsed, not all FGC SEI symbols would have been
decoded semantically. This commit detects and returns an error
in this situation.

Bug = ossfuzz:65418
Test: mvc_dec_fuzzer

decoder/ih264d_sei.c

@@ -851,6 +851,18 @@ WORD32 ih264d_parse_fgc(dec_bit_stream_t *ps_bitstrm, dec_struct_t *ps_dec,
                     return ERROR_INV_SEI_FGC_PARAMS;
                 }
 
+                /* Although the fag end of both the NALU and the bitstream buffer */
+                /* is being parsed, not all FGC SEI symbols would have been */
+                /* decoded semantically. The code below detects this condition */
+                if((ps_bitstrm->u4_ofst +
+                    ps_sei->s_sei_fgc_params.au1_num_intensity_intervals_minus1[c] *
+                        (8 + 8 +
+                         (ps_sei->s_sei_fgc_params.au1_num_model_values_minus1[c] + 1) * 16)) >=
+                   ps_bitstrm->u4_max_ofst)
+                {
+                    return ERROR_INV_SEI_FGC_PARAMS;
+                }
+
                 for(i = 0; i <= ps_sei->s_sei_fgc_params.au1_num_intensity_intervals_minus1[c]; i++)
                 {
                     ps_sei->s_sei_fgc_params.au1_intensity_interval_lower_bound[c][i] =
@@ -863,6 +875,7 @@ WORD32 ih264d_parse_fgc(dec_bit_stream_t *ps_bitstrm, dec_struct_t *ps_dec,
                     {
                         ps_sei->s_sei_fgc_params.ai4_comp_model_value[c][i][j] =
                             (WORD32) ih264d_sev(pu4_bitstrm_ofst, pu4_bitstrm_buf);
+
                         if(0 == ps_sei->s_sei_fgc_params.u1_film_grain_model_id)
                         {
                             if((1 == j) || (2 == j))