shared/install-printf: fix memleak

instance might be "", and that string would be leaked.

CID #1368264.

src/shared/install-printf.c

@@ -93,9 +93,9 @@ static int specifier_instance(char specifier, void *data, void *userdata, char *
                 return r;
 
         if (isempty(instance)) {
-                instance = strdup(i->default_instance ?: "");
-                if (!instance)
-                        return -ENOMEM;
+                r = free_and_strdup(&instance, i->default_instance ?: "");
+                if (r < 0)
+                        return r;
         }
 
         *ret = instance;