avcodec/h264: Check *log2_weight_denom

Fixes undefined behavior
Fixes: signal_sigsegv_14768d2_2248_cov_3629497219_h264_h264___pi_20070614T182942.h264
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.c

@@ -991,6 +991,16 @@ int ff_pred_weight_table(H264Context *h)
     h->luma_log2_weight_denom = get_ue_golomb(&h->gb);
     if (h->sps.chroma_format_idc)
         h->chroma_log2_weight_denom = get_ue_golomb(&h->gb);
+
+    if (h->luma_log2_weight_denom > 7U) {
+        av_log(h->avctx, AV_LOG_ERROR, "luma_log2_weight_denom %d is out of range\n", h->luma_log2_weight_denom);
+        h->luma_log2_weight_denom = 0;
+    }
+    if (h->chroma_log2_weight_denom > 7U) {
+        av_log(h->avctx, AV_LOG_ERROR, "chroma_log2_weight_denom %d is out of range\n", h->chroma_log2_weight_denom);
+        h->chroma_log2_weight_denom = 0;
+    }
+
     luma_def   = 1 << h->luma_log2_weight_denom;
     chroma_def = 1 << h->chroma_log2_weight_denom;