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

shared/config-parser.c

@@ -262,6 +262,8 @@ weston_config_get_section(struct weston_config *config, const char *section,
 	struct weston_config_section *s;
 	struct weston_config_entry *e;
 
+	if (config == NULL)
+		return NULL;
 	wl_list_for_each(s, &config->section_list, link) {
 		if (strcmp(s->name, section) != 0)
 			continue;