config-parser: Avoid null dereference when exiting. (case: weston starts without config file.)
config-parser: Avoid null dereference when exiting. (case: weston starts without config file.)

backtrace:
 (gdb) bt
 #0  0xb7704424 in __kernel_vsyscall ()
 #1  0xb757ddde in raise (sig=5) at ../nptl/sysdeps/unix/sysv/linux/pt-raise.c:42
 #2  <signal handler called>
 #3  weston_config_destroy (config=0x0) at config-parser.c:508
 #4  0xb75cbc0e in x11_destroy (ec=0x93506b0) at compositor-x11.c:1473
 #5  0x0804e0e9 in main (argc=1, argv=0xbffe5354) at compositor.c:3337

xorgy/weston,eyolfson/weston,xorgy/weston,kwm81/weston,sir-murray/weston,krezovic/weston,udoprog/weston,Gnurou/weston,Gnurou/weston,kwm81/weston,mchalupa/weston,jonnylamb/weston,sir-murray/weston,giucam/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,Tarnyko/weston-xdg_surface_present,Fantu/compositor-spice,mchalupa/weston,giucam/weston,udoprog/weston,jonnylamb/weston,Fantu/compositor-spice,krezovic/weston,eyolfson/weston