basic: introduce *_to_string_with_check() functions

They are used in later commits.

src/basic/process-util.h

@@ -137,6 +137,13 @@ static inline bool pid_is_valid(pid_t p) {
         return p > 0;
 }
 
+static inline int sched_policy_to_string_alloc_with_check(int n, char **s) {
+        if (!sched_policy_is_valid(n))
+                return -EINVAL;
+
+        return sched_policy_to_string_alloc(n, s);
+}
+
 int ioprio_parse_priority(const char *s, int *ret);
 
 pid_t getpid_cached(void);