Adds fbounds annotations for VP8LColorCache.

Reasoning:

Analysis of `VP8LColorCacheInit`
(src/utils/color_cache_utils.c:29) revealed that `colors` is allocated
using `WebPSafeCalloc` with a size of `1 << hash_bits`. Therefore,
`colors` was annotated with `WEBP_COUNTED_BY_OR_NULL(1u << hash_bits)`.
To support this, `WebPSafeCalloc` (src/utils/utils.h:59,
src/utils/utils.c:214) was annotated to return
`WEBP_SIZED_BY_OR_NULL(nmemb * size)`. Since `WebPSafeCalloc` returns a
local pointer that defaults to unsafe when bounds safety is suppressed,
`WEBP_UNSAFE_FORGE_BIDI_INDEXABLE` was used on the return value
(src/utils/utils.c:222). Similarly, `VP8LColorCacheInit` required
`WEBP_UNSAFE_FORGE_BIDI_INDEXABLE` when assigning the allocated pointer
to the struct field (src/utils/color_cache_utils.c:47). Finally,
`VP8LColorCacheInit` and `VP8LColorCacheClear` were modified to perform
side-by-side assignments to `colors` and `hash_bits` as required by the
`WEBP_COUNTED_BY_OR_NULL` annotation, using self-assignment for
`hash_bits` when necessary to maintain functional equivalence with the
original code.

Bug: 432511821
Change-Id: I63cb46909d883a2e8932043ac3117b05b37e8d40

src/utils/utils.h

@@ -56,7 +56,8 @@ static WEBP_INLINE int CheckSizeOverflow(uint64_t size) {
 WEBP_EXTERN void* WebPSafeMalloc(uint64_t nmemb, size_t size);
 // Note that WebPSafeCalloc() expects the second argument type to be 'size_t'
 // in order to favor the "calloc(num_foo, sizeof(foo))" pattern.
-WEBP_EXTERN void* WebPSafeCalloc(uint64_t nmemb, size_t size);
+WEBP_EXTERN void* WEBP_SIZED_BY_OR_NULL(nmemb* size)
+    WebPSafeCalloc(uint64_t nmemb, size_t size);
 
 // Companion deallocation function to the above allocations.
 WEBP_EXTERN void WebPSafeFree(void* const ptr);