Revert "Work around nullptr-with-nonzero-offset warning"

This reverts commit c27fa39904.

lib/decompress/zstd_decompress.c

@@ -1548,12 +1548,6 @@ size_t ZSTD_decompressBegin(ZSTD_DCtx* dctx)
     dctx->stage = ZSTDds_getFrameHeaderSize;
     dctx->processedCSize = 0;
     dctx->decodedSize = 0;
-    /* Set to non-null because ZSTD_prefetchMatch() may end up doing addition
-     * with this value for corrupted frames. However, it then just passes the
-     * pointer to PREFETCH_L1(), which doesn't require valid pointers. But,
-     * if it is NULL we get nullptr-with-nonzero-offset UBSAN warnings.
-     */
-    dctx->previousDstEnd = "";
     dctx->previousDstEnd = NULL;
     dctx->prefixStart = NULL;
     dctx->virtualStart = NULL;