[Pidgin] #3489: Fix crash due to gtkconv->{win, make_sound} dereference without gtkconv NULL-check

[Pidgin]

#3489: Fix crash due to gtkconv->{win,make_sound} dereference without gtkconv
NULL-check
--------------------------+-------------------------------------------------
 Reporter:  nix_nix       |       Type:  patch
   Status:  new           |   Priority:  minor
Component:  pidgin (gtk)  |    Version:  2.2.1
 Keywords:                |    Pending:  0    
--------------------------+-------------------------------------------------
 In gtksound.c, when deciding whether to play a sound for a conversation we
 get a PidginConversation from the PurpleConversation, because we need
 gtkconv->make_sound. However, nowadays it seems that
 PIDGIN_CONVERSATION(conv) can sometimes be NULL. Thus, we need to add a
 NULL-check.
 There was also a useless retrieval of gtkconv->win that this patch remove.

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Ticket URL: <https://developer.pidgin.im/ticket/3489>
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