restore: fix Zstandard frame handling

The snapdc tile is not conformant to the Zstandard spec.
Namely, Zstandard compressors are supposed to accept arbitrary
concatenation of frames and ignore skippable frames.

Fixes a bug where the snapshot loader aborted after the first
Zstandard frame.

Author: riptl

src/discof/restore/fd_snapdc_tile.c

@@ -176,6 +176,8 @@ handle_data_frag( fd_snapdc_tile_t *  ctx,
   uchar * out = fd_chunk_to_laddr( ctx->out.wksp, ctx->out.chunk );
 
   if( FD_UNLIKELY( !ctx->is_zstd ) ) {
+    uchar const * in = data+ctx->in.frag_pos;
+
     FD_TEST( ctx->in.frag_pos<sz );
     ulong cpy = fd_ulong_min( sz-ctx->in.frag_pos, ctx->out.mtu );
     fd_memcpy( out, in, cpy );