zstd: initialize blockProperties to avoid compiler warning

On older versions of GCC, the former code produces a compiler warning:

```
zstd/zstdlib.c: In function ‘ZSTD_decompressMultiFrame’:
zstd/zstdlib.c:38866:27: error: ‘blockProperties.origSize’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
38866 |             decodedSize = ZSTD_setRleBlock(op, (size_t)(oend-op), *ip, blockProperties.origSize);
      |                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
zstd/zstdlib.c:38849:27: note: ‘blockProperties.origSize’ was declared here
38849 |         blockProperties_t blockProperties;
      |                           ^~~~~~~~~~~~~~~
zstd/zstdlib.c:38857:9: error: ‘blockProperties’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
38857 |         switch(blockProperties.blockType)
      |         ^~~~~~
zstd/zstdlib.c:38849:27: note: ‘blockProperties’ was declared here
38849 |         blockProperties_t blockProperties;
      |                           ^~~~~~~~~~~~~~~
```

This commit works around it.

Author: indygreg

zstd/zstdlib.c

@@ -38847,6 +38847,8 @@ static size_t ZSTD_decompressFrame(ZSTD_DCtx* dctx,
     while (1) {
         size_t decodedSize;
         blockProperties_t blockProperties;
+        blockProperties.blockType = bt_reserved;
+        blockProperties.origSize = 0;
         size_t const cBlockSize = ZSTD_getcBlockSize(ip, remainingSrcSize, &blockProperties);
         if (ZSTD_isError(cBlockSize)) return cBlockSize;