mfconn_create_signed_get: add more verbosity to error conditions

mfapi/apicalls/folder_get_info.c

@@ -61,6 +61,10 @@ mfconn_api_folder_get_info(mfconn * conn, mffolder * folder,
                                                 "&response_format=json",
                                                 folderkey);
         }
+        if (api_call == NULL) {
+            fprintf(stderr, "mfconn_create_signed_get failed\n");
+            return -1;
+        }
 
         http = http_create();
         retval = http_get_buf(http, api_call, _decode_folder_get_info, folder);