Fix sentinel size miscalculation

Since is_nul() style wild-comparing stride is U64, we must add sizeof(U64) to the sentinel.

programs/lz4io.c

@@ -667,7 +667,7 @@ static unsigned long long decodeLZ4S(FILE* finput, FILE* foutput)
     inBuffSize = outBuffSize + 4;
     inBuff = (char*)malloc(inBuffSize);
 #if defined(LZ4IO_ENABLE_SPARSE_FILE)
-    outBuff = (char*)malloc(outBuffSize+1);
+    outBuff = (char*)malloc(outBuffSize+sizeof(U64));
     outBuff[outBuffSize] = 1; /* sentinel */
 #else /* LZ4IO_ENABLE_SPARSE_FILE */
     outBuff = (char*)malloc(outBuffSize);