if-options: restrict nameless option defines to encap

Nameless OT_OPTION entries are only meaningful inside an encap block;
a nameless top-level define/embed is unreferenceable and leaves
opt->var NULL, which later crashes print_option (%s) and the embedded
lookup (strcmp). Require a variable name outside O_ENCAP for both the
absent-name and empty-name cases.

Author: iliasabk

src/if-options.c

@@ -2106,7 +2106,7 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 		}
 		/* variable */
 		if (!fp) {
-			if (!(t & OT_OPTION)) {
+			if (!(t & OT_OPTION) || opt != O_ENCAP) {
 				logerrx("type %s requires a variable name",
 				    arg);
 				return -1;
@@ -2118,7 +2118,7 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 			if (fp)
 				*fp++ = '\0';
 			if (arg == NULL) {
-				if (!(t & OT_OPTION)) {
+				if (!(t & OT_OPTION) || opt != O_ENCAP) {
 					logerrx("type requires a variable name");
 					return -1;
 				}