I fell in a case where in xcb_icccm_get_wm_class_from_reply, the reply was empty, i.e., in following snippet from icccm.c, len was 0, and reply->length was 0 also.

  int len = xcb_get_property_value_length(prop->_reply);
  if(len < reply->length * 4)
    prop->instance_name[len] = 0;
  else
    prop->instance_name[len-1] = 0;

in this case, prop->instance_name[-1] is set to 0, and the following strlen(prop->instance_name) may access invalid memory.

This may be reproduced under openbox window manager, under which there is this window triggering the bug :

If xwininfo is compiled without icccm :
./xwininfo -tree -root | grep Openbox
     0xe0008b "Openbox": ("" (none))  1x1+-100+-100  +-100+-100

If xwininfo is compiled with icccm :
./xwininfo -tree -root | grep Openbox                                                                          
     0xe0008b "Openbox": ("" "")  1x1+-100+-100  +-100+-100

The second case triggers the Invalid memory access