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/idec.c

@@ -127,6 +127,7 @@ static int AppendToMemBuffer(WebPIDecoder* const idec,
   memcpy(mem->buf_ + mem->end_, data, data_size);
   mem->end_ += data_size;
   assert(mem->end_ <= mem->buf_size_);
+  assert(last_part >= 0);
   dec->parts_[last_part].buf_end_ = mem->buf_ + mem->end_;
 
   // note: setting up idec->io_ is only really needed at the beginning
@@ -155,6 +156,7 @@ static int RemapMemBuffer(WebPIDecoder* const idec,
       REMAP(dec->parts_[p].buf_end_, base, data);
     }
   }
+  assert(last_part >= 0);
   dec->parts_[last_part].buf_end_ = data + data_size;
 
   // Remap partition #0 data pointer to new offset.

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;
 }