util: fix cppcheck warnings

util.c

@@ -212,8 +212,8 @@ static int parse_native_format(const cfg_t *cfg, const char *username,
                                FILE *opwfile, device_t *devices,
                                unsigned *n_devs) {
 
-  char *s_user, *s_credential;
-  char *buf = NULL;
+  const char *s_user;
+  char *buf = NULL, *s_credential;
   size_t bufsiz = 0;
   ssize_t len;
   unsigned i;
@@ -786,7 +786,6 @@ void free_devices(device_t *devices, const unsigned n_devs) {
   }
 
   free(devices);
-  devices = NULL;
 }
 
 static int get_authenticators(const cfg_t *cfg, const fido_dev_info_t *devlist,
@@ -885,13 +884,14 @@ static void parse_opts(const cfg_t *cfg, const char *attr, struct opts *opts) {
 
 static int get_device_opts(fido_dev_t *dev, int *pin, int *uv) {
   fido_cbor_info_t *info = NULL;
-  char *const *ptr;
   const bool *val;
-  size_t len;
 
   *pin = *uv = -1; /* unsupported */
 
   if (fido_dev_is_fido2(dev)) {
+    char *const *ptr;
+    size_t len;
+
     if ((info = fido_cbor_info_new()) == NULL ||
         fido_dev_get_cbor_info(dev, info) != FIDO_OK) {
       fido_cbor_info_free(&info);
@@ -1423,7 +1423,7 @@ int do_manual_authentication(const cfg_t *cfg, const device_t *devices,
 
     debug_dbg(cfg, "Challenge: %s", b64_challenge);
 
-    n = snprintf(prompt, sizeof(prompt), "Challenge #%d:", i + 1);
+    n = snprintf(prompt, sizeof(prompt), "Challenge #%u:", i + 1);
     if (n <= 0 || (size_t) n >= sizeof(prompt)) {
       debug_dbg(cfg, "Failed to print challenge prompt");
       goto out;
@@ -1449,7 +1449,7 @@ int do_manual_authentication(const cfg_t *cfg, const device_t *devices,
            "paste the results in the prompt below.");
 
   for (i = 0; i < n_devs; ++i) {
-    n = snprintf(prompt, sizeof(prompt), "Response #%d: ", i + 1);
+    n = snprintf(prompt, sizeof(prompt), "Response #%u: ", i + 1);
     if (n <= 0 || (size_t) n >= sizeof(prompt)) {
       debug_dbg(cfg, "Failed to print response prompt");
       goto out;