svq3: Do initialization after parsing the extradata

If done before, some parameters aren't known yet.

With svq3/rtp, initializing before some parameters are known
can lead to calling av_malloc(0), which on OS X currently returns
broken pointers.

libavcodec/svq3.c

@@ -804,20 +804,11 @@ static av_cold int svq3_decode_init(AVCodecContext *avctx)
     avctx->pix_fmt = avctx->codec->pix_fmts[0];
 
     if (!s->context_initialized) {
-        s->width  = avctx->width;
-        s->height = avctx->height;
         h->halfpel_flag      = 1;
         h->thirdpel_flag     = 1;
         h->unknown_svq3_flag = 0;
         h->chroma_qp[0]      = h->chroma_qp[1] = 4;
 
-        if (MPV_common_init(s) < 0)
-            return -1;
-
-        h->b_stride = 4*s->mb_width;
-
-        ff_h264_alloc_tables(h);
-
         /* prowl for the "SEQH" marker in the extradata */
         extradata = (unsigned char *)avctx->extradata;
         for (m = 0; m < avctx->extradata_size; m++) {
@@ -904,6 +895,16 @@ static av_cold int svq3_decode_init(AVCodecContext *avctx)
 #endif
             }
         }
+
+        s->width  = avctx->width;
+        s->height = avctx->height;
+
+        if (MPV_common_init(s) < 0)
+            return -1;
+
+        h->b_stride = 4*s->mb_width;
+
+        ff_h264_alloc_tables(h);
     }
 
     return 0;