Check if new keys being set are the correct length, since longer or shorter keys yield inconsistant results

tool/yubico-piv-tool.c

@@ -1169,6 +1169,7 @@ int main(int argc, char *argv[]) {
         break;
       case action_arg_setMINUS_mgmMINUS_key:
         if(args_info.new_key_arg) {
+          if(strlen(args_info.new_key_arg) == (KEY_LEN * 2)){
             unsigned char new_key[KEY_LEN];
             size_t new_key_len = sizeof(new_key);
             if(ykpiv_hex_decode(args_info.new_key_arg, strlen(args_info.new_key_arg), new_key, &new_key_len) != YKPIV_OK) {
@@ -1178,6 +1179,9 @@ int main(int argc, char *argv[]) {
             } else {
               printf("Successfully set new management key.\n");
             }
+          } else {
+            ret = EXIT_FAILURE;
+          } 
         } else {
           fprintf(stderr, "The set-mgm-key action needs the new-key (-n) argument.\n");
           ret = EXIT_FAILURE;