Fix execSequence wildcopy undefined behavior

execSequence relied on pointer overflow to handle cases where
`sequence.matchLength < 8`.  Instead of passing an `size_t` to
wildcopy, pass a `ptrdiff_t`.

lib/common/zstd_internal.h

@@ -147,7 +147,7 @@ static void ZSTD_copy8(void* dst, const void* src) { memcpy(dst, src, 8); }
 /*! ZSTD_wildcopy() :
 *   custom version of memcpy(), can copy up to 7 bytes too many (8 bytes if length==0) */
 #define WILDCOPY_OVERLENGTH 8
-MEM_STATIC void ZSTD_wildcopy(void* dst, const void* src, size_t length)
+MEM_STATIC void ZSTD_wildcopy(void* dst, const void* src, ptrdiff_t length)
 {
     const BYTE* ip = (const BYTE*)src;
     BYTE* op = (BYTE*)dst;