idec: fix internal state corruption

A call to Append/Update would index the parts_ array w/-1 as num_parts_
had yet to be set by DecodePartition0. This would cause corruption
within the VP8Decoder member.

Fixes issue #106.

Change-Id: Ib9f2811594ff19e948a66fda862a4e0a384bb9aa

src/dec/vp8.c

@@ -47,6 +47,7 @@ VP8Decoder* VP8New(void) {
     SetOk(dec);
     WebPWorkerInit(&dec->worker_);
     dec->ready_ = 0;
+    dec->num_parts_ = 1;
   }
   return dec;
 }