WebP Decoder: Mark some truncated bitstreams as invalid

Specifically, check for truncated RIFF and/or VP8(L) chunks.

For more context, see:
https://code.google.com/p/webp/issues/detail?id=185

Change-Id: I91ca2dbf05080660fbc513244fc53adc57fc04b5

src/dec/idec.c

@@ -308,6 +308,7 @@ static VP8StatusCode DecodeWebPHeaders(WebPIDecoder* const idec) {
 
   headers.data = data;
   headers.data_size = curr_size;
+  headers.have_all_data = 0;
   status = WebPParseHeaders(&headers);
   if (status == VP8_STATUS_NOT_ENOUGH_DATA) {
     return VP8_STATUS_SUSPENDED;  // We haven't found a VP8 chunk yet.