liblzma: Fix copying of check type statistics in lzma_index_cat().

The check type of the last Stream in dest was never copied to
dest->checks (the code tried to copy it but it was done too late).
This meant that the value returned by lzma_index_checks() would
only include the check type of the last Stream when multiple
lzma_indexes had been concatenated.

In xz --list this meant that the summary would only list the
check type of the last Stream, so in this sense this was only
a visual bug. However, it's possible that some applications
use this information for purposes other than merely showing
it to the users in an informational message. I'm not aware of
such applications though and it's quite possible that such
applications don't exist.

Regular streamed decompression in xz or any other application
doesn't use lzma_index_cat() and so this bug cannot affect them.

src/liblzma/common/index.c

@@ -839,6 +839,11 @@ lzma_index_cat(lzma_index *restrict dest, lzma_index *restrict src,
 		}
 	}
 
+	// dest->checks includes the check types of all except the last Stream
+	// in dest. Set the bit for the check type of the last Stream now so
+	// that it won't get lost when Stream(s) from src are appended to dest.
+	dest->checks = lzma_index_checks(dest);
+
 	// Add all the Streams from src to dest. Update the base offsets
 	// of each Stream from src.
 	const index_cat_info info = {
@@ -855,7 +860,7 @@ lzma_index_cat(lzma_index *restrict dest, lzma_index *restrict src,
 	dest->total_size += src->total_size;
 	dest->record_count += src->record_count;
 	dest->index_list_size += src->index_list_size;
-	dest->checks = lzma_index_checks(dest) | src->checks;
+	dest->checks |= src->checks;
 
 	// There's nothing else left in src than the base structure.
 	lzma_free(src, allocator);