avcodec/vp3: Clear context on reinitialization failure

Fixes null pointer dereference
Fixes: 1536b9b096a8f95b742bae9d3d761cc6/signal_sigsegv_294aaed_2039_8d1797aeb823ea43858d0fa45c9eb899.ogv

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/vp3.c

@@ -2018,17 +2018,19 @@ static int vp3_decode_frame(AVCodecContext *avctx,
             vp3_decode_end(avctx);
             ret = theora_decode_header(avctx, &gb);
 
+            if (ret >= 0)
+                ret = vp3_decode_init(avctx);
             if (ret < 0) {
                 vp3_decode_end(avctx);
-            } else
-                ret = vp3_decode_init(avctx);
+            }
             return ret;
         } else if (type == 2) {
             ret = theora_decode_tables(avctx, &gb);
+            if (ret >= 0)
+                ret = vp3_decode_init(avctx);
             if (ret < 0) {
                 vp3_decode_end(avctx);
-            } else
-                ret = vp3_decode_init(avctx);
+            }
             return ret;
         }