basic: introduce *_to_string_with_check() functions

They are used in later commits.

src/basic/securebits-util.h

@@ -24,6 +24,14 @@
 
 int secure_bits_to_string_alloc(int i, char **s);
 int secure_bits_from_string(const char *s);
+
 static inline bool secure_bits_is_valid(int i) {
         return ((SECURE_ALL_BITS | SECURE_ALL_LOCKS) & i) == i;
 }
+
+static inline int secure_bits_to_string_alloc_with_check(int n, char **s) {
+        if (!secure_bits_is_valid(n))
+                return -EINVAL;
+
+        return secure_bits_to_string_alloc(n, s);
+}