segfault on exporting changelog

[cebe]

while testing #582 I found that when I start utox and go to friend settings of an offline firend for which I do not have a chat history, I get the following ASAN segfaul:

$ ASAN_SYMBOLIZER_PATH=/usr/bin/llvm-symbolizer-3.5 ./utox -p
AL lib: (WW) alcCloseDevice: Releasing context 0x613000045400
ASAN:SIGSEGV
=================================================================
==29069==ERROR: AddressSanitizer: SEGV on unknown address 0x0000dabfb229 (pc 0x000000817b0d bp 0x7fcc91a34e90 sp 0x7fcc91a34b80 T11)
    #0 0x817b0c in ugtk_save_chatlog_thread /home/cebe/dev/tox/uTox/src/xlib/gtk.c:305:5
    #1 0x7fccaa1d60a3 in start_thread /build/glibc-daoqzt/glibc-2.19/nptl/pthread_create.c:309
    #2 0x7fcca7c4662c in clone /build/glibc-daoqzt/glibc-2.19/misc/../sysdeps/unix/sysv/linux/x86_64/clone.S:111

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/cebe/dev/tox/uTox/src/xlib/gtk.c:305 ugtk_save_chatlog_thread
Thread T11 created by T0 here:
    #0 0x499b3f in __interceptor_pthread_create (/home/cebe/dev/tox/uTox/build-cmake-debug/utox+0x499b3f)
    #1 0x7f1fb4 in thread /home/cebe/dev/tox/uTox/src/xlib/main.c:151:5
    #2 0x817777 in ugtk_save_chatlog /home/cebe/dev/tox/uTox/src/xlib/gtk.c:368:5
    #3 0x807a83 in native_export_chatlog_init /home/cebe/dev/tox/uTox/src/xlib/filesys.c:127:9
    #4 0x4d6ba1 in utox_export_chatlog_init /home/cebe/dev/tox/uTox/src/chatlog.c:213:5
    #5 0x82b9cd in button_export_chatlog_on_mup /home/cebe/dev/tox/uTox/src/ui/buttons.c:424:5
    #6 0x823abc in button_mup /home/cebe/dev/tox/uTox/src/ui/button.c:122:13
    #7 0x69878e in panel_mup /home/cebe/dev/tox/uTox/src/ui.c:591:30
    #8 0x698a57 in panel_mup /home/cebe/dev/tox/uTox/src/ui.c:596:25
    #9 0x698a57 in panel_mup /home/cebe/dev/tox/uTox/src/ui.c:596:25
    #10 0x698a57 in panel_mup /home/cebe/dev/tox/uTox/src/ui.c:596:25
    #11 0x698a57 in panel_mup /home/cebe/dev/tox/uTox/src/ui.c:596:25
    #12 0x698a57 in panel_mup /home/cebe/dev/tox/uTox/src/ui.c:596:25
    #13 0x80e1e7 in doevent /home/cebe/dev/tox/uTox/src/xlib/event.c:291:25
    #14 0x80024e in main /home/cebe/dev/tox/uTox/src/xlib/main.c:950:14
    #15 0x7fcca7b7fb44 in __libc_start_main /build/glibc-daoqzt/glibc-2.19/csu/libc-start.c:287

==29069==ABORTING