cgit.c: do not segfault on unexpected query-string format

The querystring_cb() function will be invoked with a NULL value when
the querystring contains a name not followed by a '='. Such a value
used to cause a segfault, which this patch fixes.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.c

@@ -132,6 +132,9 @@ void config_cb(const char *name, const char *value)
 
 static void querystring_cb(const char *name, const char *value)
 {
+	if (!value)
+		value = "";
+
 	if (!strcmp(name,"r")) {
 		ctx.qry.repo = xstrdup(value);
 		ctx.repo = cgit_get_repoinfo(value);