if-options: fix NULL dereference on missing variable name

strskipwhite() returns NULL when the remainder of a define/define6/
definend line is empty or whitespace-only (e.g. a trailing escaped
space protected from the config trim). parse_option() then called
strcasecmp(NULL, "reserved") and crashed.

Treat a NULL variable name the same as an absent one: error out for
types that require a name and accept np == NULL for OT_OPTION.

Reproducer: a config line 'define 119 string=x\\ ' (escaped trailing
space) segfaults at strcasecmp in parse_option (if-options.c:2120).

Fixes #731.

Author: iliasabk

src/if-options.c

@@ -2117,7 +2117,13 @@ parse_option(struct dhcpcd_ctx *ctx, const char *ifname, struct if_options *ifo,
 			fp = strwhite(arg);
 			if (fp)
 				*fp++ = '\0';
-			if (strcasecmp(arg, "reserved")) {
+			if (arg == NULL) {
+				if (!(t & OT_OPTION)) {
+					logerrx("type requires a variable name");
+					return -1;
+				}
+				np = NULL;
+			} else if (strcasecmp(arg, "reserved")) {
 				np = strdup(arg);
 				if (np == NULL) {
 					logerr(__func__);