Check for NULLs params within the functions
As assumed in the prefs_themes_done() calls. May fix some strange crash
when exiting Claws Mail in Debian's version 3.14.1-3+b1:
*** Error in `/usr/bin/claws-mail': munmap_chunk(): invalid pointer:
0x00000000026a6f80 ***
======= Backtrace: =========
/lib/x86_64-linux-gnu/libc.so.6(+0x70bcb)[0x7fa85407cbcb]
/lib/x86_64-linux-gnu/libc.so.6(+0x76f96)[0x7fa854082f96]
/usr/bin/claws-mail(stock_pixmap_themes_list_free+0x28)[0x55a7a8]
/usr/bin/claws-mail(prefs_themes_done+0x3a)[0x5389ba]
/usr/bin/claws-mail[0x449329]
/usr/bin/claws-mail(main+0x2326)[0x44b986]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf1)[0x7fa85402c2b1]
/usr/bin/claws-mail(_start+0x2a)[0x44bfda]