Fix crash when projectsfile cannot be opened

This patch makes cgit properly abort in case the projectsfile cannot
be opened. Without the added return cgit continues using the projects
pointer which is NULL and thus causes a segfault.

scan-tree.c

@@ -219,6 +219,7 @@ void scan_projects(const char *path, const char *projectsfile, repo_config_fn fn
 	if (!projects) {
 		fprintf(stderr, "Error opening projectsfile %s: %s (%d)\n",
 			projectsfile, strerror(errno), errno);
+		return;
 	}
 	while (fgets(line, sizeof(line), projects) != NULL) {
 		for (z = &lastc(line);