liblzma: Don't create an empty Block in lzma_stream_buffer_encode().

Empty Block was created if the input buffer was empty.
Empty Block wastes a few bytes of space, but more importantly
it triggers a bug in XZ Utils 5.0.1 and older when trying
to decompress such a file. 5.0.1 and older consider such
files to be corrupt. I thought that no encoder creates empty
Blocks when releasing 5.0.2 but I was wrong.

src/liblzma/common/stream_buffer_encoder.c

@@ -84,24 +84,30 @@ lzma_stream_buffer_encode(lzma_filter *filters, lzma_check check,
 
 	out_pos += LZMA_STREAM_HEADER_SIZE;
 
-	// Block
+	// Encode a Block but only if there is at least one byte of input.
 	lzma_block block = {
 		.version = 0,
 		.check = check,
 		.filters = filters,
 	};
 
+	if (in_size > 0)
 		return_if_error(lzma_block_buffer_encode(&block, allocator,
 				in, in_size, out, &out_pos, out_size));
 
 	// Index
 	{
-		// Create an Index with one Record.
+		// Create an Index. It will have one Record if there was
+		// at least one byte of input to encode. Otherwise the
+		// Index will be empty.
 		lzma_index *i = lzma_index_init(allocator);
 		if (i == NULL)
 			return LZMA_MEM_ERROR;
 
-		lzma_ret ret = lzma_index_append(i, allocator,
+		lzma_ret ret = LZMA_OK;
+
+		if (in_size > 0)
+			ret = lzma_index_append(i, allocator,
 					lzma_block_unpadded_size(&block),
 					block.uncompressed_size);