[Pidgin] #9120: Crash when changing buddy icon with mystatusbox enabled

[Pidgin]

#9120: Crash when changing buddy icon with mystatusbox enabled
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 Reporter:  darkrain42  |     Owner:  darkrain42
     Type:  defect      |    Status:  new       
Component:  libpurple   |   Version:  2.5.5     
 Keywords:  statusbox   |  
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 The imgstore/buddy icons code doesn't handle references to the
 `PurpleStoredImage` properly in `purple_buddy_icons_set_account_icon()`,
 which leads to a crash when trying to change avatars with mystatusbox
 enabled (an image is freed and then used).

 Reported to Ubuntu LP at
 https://bugs.launchpad.net/ubuntu/+source/pidgin/+bug/371898

 I'm not 100% sure, but it looks like the `pointer_icon_cache` hash table
 in buddyicon.c is supposed to own a reference to all the images stored
 there (based on the comments about !StoredImage references in the header),
 but not all the code paths are doing that properly
 (purple_buddy_icons_find_account_icon,
 purple_buddy_icons_node_find_custom_icon,
 purple_buddy_icons_node_find_custom_icon don't when loading from disk).

 Also, it seems like pointer_icon_cache should have the destroy func set to
 purple_imgstore_unref (again not sure, just gut).

 Attaching the work-in-progress (this fixes the crash, but I think I'm
 still leaking !StoredImage references and the above issues haven't been
 dealt with). Patch is against head-ish

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Ticket URL: <http://developer.pidgin.im/ticket/9120>
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