[linux] Port libzstd UBSAN fix

contrib/linux-kernel/lib/zstd/compress.c

@@ -1978,10 +1978,15 @@ void ZSTD_compressBlock_lazy_generic(ZSTD_CCtx *ctx, const void *src, size_t src
 				break; /* nothing found : store previous solution */
 			}
 
+		/* NOTE:
+		 * start[-offset+ZSTD_REP_MOVE-1] is undefined behavior.
+		 * (-offset+ZSTD_REP_MOVE-1) is unsigned, and is added to start, which
+		 * overflows the pointer, which is undefined behavior.
+		 */
 		/* catch up */
 		if (offset) {
 			while ((start > anchor) && (start > base + offset - ZSTD_REP_MOVE) &&
-			       (start[-1] == start[-1 - offset + ZSTD_REP_MOVE])) /* only search for offset within prefix */
+			       (start[-1] == (start-offset+ZSTD_REP_MOVE)[-1])) /* only search for offset within prefix */
 			{
 				start--;
 				matchLength++;