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/legacy/zstd_v02.c

@@ -2808,7 +2808,7 @@ static void   ZSTD_copy8(void* dst, const void* src) { memcpy(dst, src, 8); }
 #define COPY8(d,s) { ZSTD_copy8(d,s); d+=8; s+=8; }
 
 /*! ZSTD_wildcopy : custom version of memcpy(), can copy up to 7-8 bytes too many */
-static void ZSTD_wildcopy(void* dst, const void* src, size_t length)
+static void ZSTD_wildcopy(void* dst, const void* src, ptrdiff_t length)
 {
     const BYTE* ip = (const BYTE*)src;
     BYTE* op = (BYTE*)dst;
@@ -3218,7 +3218,7 @@ static size_t ZSTD_execSequence(BYTE* op,
         }
         else
         {
-            ZSTD_wildcopy(op, match, sequence.matchLength-8);   /* works even if matchLength < 8 */
+            ZSTD_wildcopy(op, match, (ptrdiff_t)sequence.matchLength-8);   /* works even if matchLength < 8 */
         }
     }