Created attachment 8223 [details]
data that causes jbig2dec to null-pointer dereference

NULL-pointer dereference at jbig2_image_clone in
jbig2_image.c:61 with current git version (c29b63557cb191f from )

It occurs if jbig2dec is called with <global_stream> <page_stream> and
no page segment with image description is inside <page_stream> (for example if global and page streams are exchanged). 
Function jbig2_page_out tries to jbig2_image_clone with a null-pointer.

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Reading symbols from /home/user/sem12/ma/pdf/jbig2/jbig2dec_git/jbig2dec/.libs/jbig2dec...done.
(gdb) r -o output.pbm page_seg global_seg 
Starting program: /home/user/sem12/ma/pdf/jbig2/jbig2dec_git/jbig2dec/.libs/jbig2dec -o output.pbm page_seg global_seg

Program received signal SIGSEGV, Segmentation fault.
0x00007ffff7bd79d9 in jbig2_image_clone (ctx=0x607d60, image=0x0) at jbig2_image.c:63
63              image->refcount++;
(gdb) bt
#0  0x00007ffff7bd79d9 in jbig2_image_clone (ctx=0x607d60, image=0x0) at jbig2_image.c:63
#1  0x00007ffff7bcda07 in jbig2_page_out (ctx=0x607d60) at jbig2_page.c:297
#2  0x0000000000401dc1 in main (argc=5, argv=0x7fffffffd938) at jbig2dec.c:497

the used streams (page_seg and global_seg) are attached. 

This patch should fix the issue:

diff --git a/jbig2_image.c b/jbig2_image.c
index 7037d71..45db3b3 100644
--- a/jbig2_image.c
+++ b/jbig2_image.c
@@ -60,7 +60,9 @@ Jbig2Image* jbig2_image_new(Jbig2Ctx *ctx, int width, int height)
 /* clone an image pointer by bumping its reference count */
 Jbig2Image* jbig2_image_clone(Jbig2Ctx *ctx, Jbig2Image *image)
 {
-	image->refcount++;
+	if (image != NULL)
+		image->refcount++;
+
 	return image;
 }