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Description of problem:
gphoto2 --sumary segfaults when there's no storage card in any Canon
family camera. (Canon EOS 300D in my case)

What happens:
-----------------------------------------------------------------------
env LANG=C gdb --args gphoto2 --debug --summary

<snip>

10.597199 canon/usb.c(2): canon_usb_dialogue: camera status "No storage
card in camera" in response to command 0x9 0x11 0x201 (Disk info
request)

Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread -1208162640 (LWP 3774)]
canon_int_get_disk_name_info (camera=0x8c71860, name=0xbfbe368e "C:\\",
capacity=0x8c754f8, available=0x8c754fc, context=0x8c74840) at
canon.c:1788
1788                                    cap = le32atoh (msg + 4);
(gdb) p msg
$1 = (unsigned char *) 0x0
-----------------------------------------------------------------------

The returned msg from the canon_usb_dialogue() is used before checked
it's != NULL. It causes the segfault when there's no storage card in the
camera.

Version-Release number of selected component (if applicable):


How reproducible:
Always

Steps to Reproduce:
1. Have a Canon camera connected via USB.
2. gphoto2 --summary
3. Segmentation Fault
  

Actual Results:  Segmentation Fault

Expected Results:  No Segmentation Fault.

Additional info: