Return LZMA_HEADER_ERROR if LZMA_SYNC_FLUSH is used with any

of the so called simple filters. If there is demand, limited
support for LZMA_SYNC_FLUSH may be added in future.

After this commit, using LZMA_SYNC_FLUSH shouldn't cause
undefined behavior in any situation.

src/liblzma/api/lzma/simple.h

@@ -64,6 +64,15 @@
  *
  * If options with non-default values have been specified when encoding,
  * the same options must also be specified when decoding.
+ *
+ * \note        At the moment, none of the simple filters support
+ *              LZMA_SYNC_FLUSH. If LZMA_SYNC_FLUSH is specified,
+ *              LZMA_HEADER_ERROR will be returned. If there is need,
+ *              partial support for LZMA_SYNC_FLUSH can be added in future.
+ *              Partial means that flushing would be possible only at
+ *              offsets that are multiple of 2, 4, or 16 depending on
+ *              the filter, except x86 which cannot be made to support
+ *              LZMA_SYNC_FLUSH predictably.
  */
 typedef struct {
 	/**

src/liblzma/simple/simple_coder.c

@@ -101,6 +101,14 @@ simple_code(lzma_coder *coder, lzma_allocator *allocator,
 		size_t in_size, uint8_t *restrict out,
 		size_t *restrict out_pos, size_t out_size, lzma_action action)
 {
+	// TODO: Add partial support for LZMA_SYNC_FLUSH. We can support it
+	// in cases when the filter is able to filter everything. With most
+	// simple filters it can be done at offset that is a multiple of 2,
+	// 4, or 16. With x86 filter, it needs good luck, and thus cannot
+	// be made to work predictably.
+	if (action == LZMA_SYNC_FLUSH)
+		return LZMA_HEADER_ERROR;
+
 	// Flush already filtered data from coder->buffer[] to out[].
 	if (coder->pos < coder->filtered) {
 		bufcpy(coder->buffer, &coder->pos, coder->filtered,