parse-util: make return parameter optional in safe_atou16_full()

All other safe_atoXYZ_full() functions have the parameter optional,
let's make it optoinal here, too.

src/basic/parse-util.c

@@ -535,7 +535,9 @@ int safe_atou16_full(const char *s, unsigned base, uint16_t *ret) {
         if ((unsigned long) (uint16_t) l != l)
                 return -ERANGE;
 
-        *ret = (uint16_t) l;
+        if (ret)
+                *ret = (uint16_t) l;
+
         return 0;
 }