Fix a bug in alloc_size for high bit depths

I introduced this bug in commit 2e32276:
https://chromium-review.googlesource.com/c/webm/libvpx/+/5446333

I changed the line

  stride_in_bytes = (fmt & VPX_IMG_FMT_HIGHBITDEPTH) ? s * 2 : s;

to three lines:

  s = (fmt & VPX_IMG_FMT_HIGHBITDEPTH) ? s * 2 : s;
  if (s > INT_MAX) goto fail;
  stride_in_bytes = (int)s;

But I didn't realize that `s` is used later in the calculation of
alloc_size.

As a quick fix, undo the effect of s * 2 for high bit depths after `s`
has been assigned to stride_in_bytes.

Bug: chromium:332382766
Change-Id: I53fbf405555645ab1d7254d31aadabe4f426be8c
(cherry picked from commit 74c70af)

vpx/src/vpx_image.c

@@ -109,6 +109,7 @@ static vpx_image_t *img_alloc_helper(vpx_image_t *img, vpx_img_fmt_t fmt,
   s = (s + stride_align - 1) & ~((uint64_t)stride_align - 1);
   if (s > INT_MAX) goto fail;
   stride_in_bytes = (int)s;
+  s = (fmt & VPX_IMG_FMT_HIGHBITDEPTH) ? s / 2 : s;
 
   /* Allocate the new image */
   if (!img) {