[Backport] Security bug 1455619

Cherry-pick of patch originally reviewed on
https://chromium-review.googlesource.com/c/webm/libwebp/+/4634862:
EncodeAlphaInternal: add missing error check

VP8LBitWriterFinish() may cause the VP8LBitWriter's buffer to be grown.
If that allocation fails, VP8LBitWriterNumBytes() will return a size
larger than the current allocation resulting in a heap overwrite of the
missing bytes.

==13==ERROR: AddressSanitizer: heap-buffer-overflow on address
0x61900005b880 at pc 0x00000049ffc1 bp 0x7fff144f5b40 sp 0x7fff144f5310
READ of size 1028 at 0x61900005b880 thread T0
    #0 0x49ffc0 in __asan_memcpy
    #1 0x695861 in VP8BitWriterAppend src/utils/bit_writer_utils.c:186:3
    #2 0x65acf9 in EncodeAlphaInternal src/enc/alpha_enc.c:169:14

Found by Nallocfuzz (https://github.com/catenacyber/nallocfuzz).

This is the same issue that was fixed in the non-alpha lossless path in:
d49cfbb3 vp8l_enc,WriteImage: add missing error check

Bug: chromium:1455619
Change-Id: I6bd10de213707d3d6b7ce3d0d2b3942af45d317f
(cherry picked from commit c3bd7cff2e57b4bf1b744e70dd379570d83fb0e4)
Reviewed-on: https://codereview.qt-project.org/c/qt/qtwebengine-chromium/+/500280
Reviewed-by: Michal Klocek <[email protected]>

chromium/third_party/libwebp/src/src/enc/alpha_enc.c

@@ -140,6 +140,11 @@ 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.