Fix segmentation fault in empty repository

When a repository is empty, the ATOM feed link is written in the header,
but this involves formatting ctx->qry.head which is NULL in this case.
With glibc, vsnprintf formats "%s" with a NULL input as "(null)" but on
Solaris this results in a segmentation fault.  Since we don't have a
meaningful head for the atom feed in an empty repository, it's simplest
not to write out the link element at all.

Signed-off-by: John Keeping <john@metanate.com>

ui-shared.c

@@ -619,7 +619,7 @@ void cgit_print_docstart(struct cgit_context *ctx)
 		html_attr(ctx->cfg.favicon);
 		html("'/>\n");
 	}
-	if (host && ctx->repo) {
+	if (host && ctx->repo && ctx->qry.head) {
 		html("<link rel='alternate' title='Atom feed' href='");
 		html(cgit_httpscheme());
 		html_attr(cgit_hosturl());