Fix OOB write in BuildHuffmanTable.

First, BuildHuffmanTable is called to check if the data is valid.
If it is and the table is not big enough, more memory is allocated.

This will make sure that valid (but unoptimized because of unbalanced
codes) streams are still decodable.

Bug: chromium:1479274
Change-Id: I31c36dbf3aa78d35ecf38706b50464fd3d375741
(cherry picked from commit 902bc91903)
(cherry picked from commit 2af26267cd)

src/enc/alpha_enc.c

@@ -141,11 +141,6 @@ static int EncodeAlphaInternal(const uint8_t* const data, int width, int height,
                               !reduce_levels, &tmp_bw, &result->stats);
     if (ok) {
       output = VP8LBitWriterFinish(&tmp_bw);
-      if (tmp_bw.error_) {
-        VP8LBitWriterWipeOut(&tmp_bw);
-        memset(&result->bw, 0, sizeof(result->bw));
-        return 0;
-      }
       output_size = VP8LBitWriterNumBytes(&tmp_bw);
       if (output_size > data_size) {
         // compressed size is larger than source! Revert to uncompressed mode.