libavcodec/h264_parse: don't use uninitialized value when chroma_format_idc==0

When parsing a monochrome file, chroma_log2_weight_denom was used without
being initialized, which could lead to a bogus error message being printed, e.g.
  [h264 @ 0x61a000026480] chroma_log2_weight_denom 24576 is out of range
It also could led to warnings using AddressSanitizer.
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264_parse.c

@@ -34,21 +34,22 @@ int ff_h264_pred_weight_table(GetBitContext *gb, const SPS *sps,
 
     pwt->use_weight             = 0;
     pwt->use_weight_chroma      = 0;
-    pwt->luma_log2_weight_denom = get_ue_golomb(gb);
-    if (sps->chroma_format_idc)
-        pwt->chroma_log2_weight_denom = get_ue_golomb(gb);
 
+    pwt->luma_log2_weight_denom = get_ue_golomb(gb);
     if (pwt->luma_log2_weight_denom > 7U) {
         av_log(logctx, AV_LOG_ERROR, "luma_log2_weight_denom %d is out of range\n", pwt->luma_log2_weight_denom);
         pwt->luma_log2_weight_denom = 0;
     }
+    luma_def = 1 << pwt->luma_log2_weight_denom;
+
+    if (sps->chroma_format_idc) {
+        pwt->chroma_log2_weight_denom = get_ue_golomb(gb);
         if (pwt->chroma_log2_weight_denom > 7U) {
             av_log(logctx, AV_LOG_ERROR, "chroma_log2_weight_denom %d is out of range\n", pwt->chroma_log2_weight_denom);
             pwt->chroma_log2_weight_denom = 0;
         }
-
-    luma_def   = 1 << pwt->luma_log2_weight_denom;
         chroma_def = 1 << pwt->chroma_log2_weight_denom;
+    }
 
     for (list = 0; list < 2; list++) {
         pwt->luma_weight_flag[list]   = 0;
@@ -102,12 +103,14 @@ int ff_h264_pred_weight_table(GetBitContext *gb, const SPS *sps,
             if (picture_structure == PICT_FRAME) {
                 pwt->luma_weight[16 + 2 * i][list][0] = pwt->luma_weight[16 + 2 * i + 1][list][0] = pwt->luma_weight[i][list][0];
                 pwt->luma_weight[16 + 2 * i][list][1] = pwt->luma_weight[16 + 2 * i + 1][list][1] = pwt->luma_weight[i][list][1];
+                if (sps->chroma_format_idc) {
                     for (j = 0; j < 2; j++) {
                         pwt->chroma_weight[16 + 2 * i][list][j][0] = pwt->chroma_weight[16 + 2 * i + 1][list][j][0] = pwt->chroma_weight[i][list][j][0];
                         pwt->chroma_weight[16 + 2 * i][list][j][1] = pwt->chroma_weight[16 + 2 * i + 1][list][j][1] = pwt->chroma_weight[i][list][j][1];
                     }
                 }
             }
+        }
         if (slice_type_nos != AV_PICTURE_TYPE_B)
             break;
     }