mksquashfs: ensure block_list is always initialised in write_file_blocks_dup()

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
diff --git a/squashfs-tools/mksquashfs.c b/squashfs-tools/mksquashfs.c
index 765c681..b78c87e 100644
--- a/squashfs-tools/mksquashfs.c
+++ b/squashfs-tools/mksquashfs.c
@@ -3081,6 +3081,7 @@
 
 	for(block = 0; block < blocks;) {
 		if(read_buffer->fragment && read_buffer->c_byte) {
+			block_list[block] = 0;
 			fragment_buffer = read_buffer;
 			blocks = read_size >> block_log;
 		} else {