Do not allow empty option values except for the closing brace in struct proxyspecs

It is better to error out earlier than to pass the option down to be
rejected by its handler (the handler may fail to reject, as was the case
with a couple of options).

src/opts.c

@@ -4270,7 +4270,7 @@ set_option(opts_t *opts, const char *argv0,
 {
 	int yes;
 
-	if (!value) {
+	if (!value || !strlen(value)) {
 		fprintf(stderr, "Error in conf: No value assigned for %s on line %d\n", name, line_num);
 		return -1;
 	}
@@ -4444,6 +4444,13 @@ static int WUNRES
 set_proxyspec_option(proxyspec_t *spec, const char *argv0,
 		const char *name, char *value, char **natengine, spec_addrs_t *spec_addrs, int line_num)
 {
+	// Closing brace '}' is the only option without a value
+	// and only allowed in structured proxyspecs
+	if ((!value || !strlen(value)) && !equal(name, "}")) {
+		fprintf(stderr, "Error in conf: No value assigned for %s on line %d\n", name, line_num);
+		return -1;
+	}
+
 	if (equal(name, "Proto")) {
 		if (proxyspec_set_proto(spec, value) == -1)
 			return -1;
@@ -4716,7 +4723,7 @@ set_global_option(global_t *global, const char *argv0,
 {
 	int yes;
 
-	if (!value) {
+	if (!value || !strlen(value)) {
 		fprintf(stderr, "Error in conf: No value assigned for %s on line %d\n", name, *line_num);
 		return -1;
 	}
@@ -4797,11 +4804,8 @@ set_global_option(global_t *global, const char *argv0,
 			log_dbg_printf("ProxySpec { on line %d\n", *line_num);
 #endif /* DEBUG_OPTS */
 			return load_proxyspec_struct(global, argv0, natengine, line_num, f, tmp_global_opts);
-		} else if (strlen(value)) {
-			return load_proxyspec_line(global, argv0, value, natengine, *line_num, tmp_global_opts);
 		} else {
-			fprintf(stderr, "Incomplete ProxySpec on line %d\n", *line_num);
-			return -1;
+			return load_proxyspec_line(global, argv0, value, natengine, *line_num, tmp_global_opts);
 		}
 	} else if (equal(name, "ConnIdleTimeout")) {
 		unsigned int i = atoi(value);