[Pidgin] #10992: segfault after reinit: cache_dir not zeroed

[Pidgin]

#10992: segfault after reinit: cache_dir not zeroed
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Reporter:  cavedon  |        Type:  defect   
  Status:  new      |   Component:  libpurple
 Version:  2.6.4    |    Keywords:           
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 If you uninitialize the library and than initialized it again, you get a
 segmentation fault because the pointer cache_dir in buddyicon.c is freed
 but not set to NULL, so the libraries tries to free it again upon re-
 initialization.

 I attaching a fix for that.

 Please considering zeroing freed pointers library-wise, in order to reduce
 or help detecting such issues!

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Ticket URL: <http://developer.pidgin.im/ticket/10992>
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