Bug 1819244. Cherry pic webp commit fix. r=aosmond, a=dsmith

https://github.com/webmproject/libwebp/commit/a486d800b60d0af4cc0836bf7ed8f21e12974129

Differential Revision: https://phabricator.services.mozilla.com/D171814

media/libwebp/MOZCHANGES

 Changes made to pristine libwebp source by mozilla.org developers.
 
-2022/05/06  -- Cherry-pick upstream crash fix (bug 1761275).
+2023/03/06  -- Cherry-pick upstream fix (bug 1819244).
 
 2022/02/03  -- Enabled automatic updates via Updatebot (meta bug #1618282). For
                further updates to the library please review the mercurial

media/libwebp/src/enc/alpha_enc.c

@@ -13,6 +13,7 @@
 
 #include <assert.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include "src/enc/vp8i_enc.h"
 #include "src/dsp/dsp.h"
@@ -148,6 +149,7 @@ static int EncodeAlphaInternal(const uint8_t* const data, int width, int height,
       }
     } else {
       VP8LBitWriterWipeOut(&tmp_bw);
+      memset(&result->bw, 0, sizeof(result->bw));
       return 0;
     }
   }
@@ -162,7 +164,7 @@ static int EncodeAlphaInternal(const uint8_t* const data, int width, int height,
   header = method | (filter << 2);
   if (reduce_levels) header |= ALPHA_PREPROCESSED_LEVELS << 4;
 
-  VP8BitWriterInit(&result->bw, ALPHA_HEADER_LEN + output_size);
+  if (!VP8BitWriterInit(&result->bw, ALPHA_HEADER_LEN + output_size)) ok = 0;
   ok = ok && VP8BitWriterAppend(&result->bw, &header, ALPHA_HEADER_LEN);
   ok = ok && VP8BitWriterAppend(&result->bw, output, output_size);