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_v07.c

@@ -2845,7 +2845,7 @@ static void ZSTDv07_copy8(void* dst, const void* src) { memcpy(dst, src, 8); }
 /*! ZSTDv07_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 ZSTDv07_wildcopy(void* dst, const void* src, size_t length)
+MEM_STATIC void ZSTDv07_wildcopy(void* dst, const void* src, ptrdiff_t length)
 {
     const BYTE* ip = (const BYTE*)src;
     BYTE* op = (BYTE*)dst;
@@ -3726,7 +3726,7 @@ size_t ZSTDv07_execSequence(BYTE* op,
         }
         while (op < oMatchEnd) *op++ = *match++;
     } else {
-        ZSTDv07_wildcopy(op, match, sequence.matchLength-8);   /* works even if matchLength < 8 */
+        ZSTDv07_wildcopy(op, match, (ptrdiff_t)sequence.matchLength-8);   /* works even if matchLength < 8 */
     }
     return sequenceLength;
 }