h264: check for integer overflow, fix null pointer dereference

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -4010,6 +4010,8 @@ again:
                            "SPS decoding failure, trying again with the complete NAL\n");
                     if (h->is_avc)
                         av_assert0(next_avc - buf_index + consumed == nalsize);
+                    if ((next_avc - buf_index + consumed - 1) >= INT_MAX/8)
+                        break;
                     init_get_bits(&s->gb, &buf[buf_index + 1 - consumed],
                                   8*(next_avc - buf_index + consumed - 1));
                     ff_h264_decode_seq_parameter_set(h);