options: Ensure ldop is not NULL dereferenced (#568)

ldop itself cannot be non NULL as it points to the location.
but *ldop CAN be NULL.

Fixes #567.

Author: rsmarples

src/if-options.c

@@ -1882,7 +1882,7 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 			if (*edop) {
 				dop = &(*edop)->embopts;
 				dop_len = &(*edop)->embopts_len;
-			} else if (ldop) {
+			} else if (*ldop) {
 				dop = &(*ldop)->embopts;
 				dop_len = &(*ldop)->embopts_len;
 			} else {