config-parser: Avoid null dereference when handling config-parser
backtrace:
(gdb) bt
#0 weston_config_get_section (config=0x0, section=0x8062f31 "keyboard", key=0x0, value=0x0)
at config-parser.c:265
#1 0x080535a1 in weston_compositor_init (ec=0x905b690, display=0x9056490, argc=0xbf8bd2f0,
argv=0xbf8bd384, config_fd=-1) at compositor.c:2819
#2 0xb75d72bb in x11_compositor_create (config_fd=-1, argv=0xbf8bd384, argc=<optimized out>,
use_pixman=0, no_input=0, fullscreen=0, display=0x9056490) at compositor-x11.c:1527
#3 backend_init (display=0x9056490, argc=0xbf8bd2f0, argv=0xbf8bd384, config_fd=-1)
at compositor-x11.c:1746