Allow empty magic

src/config.rs

@@ -141,6 +141,7 @@ impl Config {
             .arg(
                 Arg::with_name("magic")
                     .long("magic")
+                    .default_value("")
                     .takes_value(true),
             )
             .arg(
@@ -336,6 +337,7 @@ impl Config {
         let network_type = Network::from(network_name);
         let magic: Option<u32> = m
             .value_of("magic")
+            .filter(|s| !s.is_empty())
             .map(|s| u32::from_str_radix(s, 16).expect("invalid network magic"));
         let db_dir = Path::new(m.value_of("db_dir").unwrap_or("./db"));
         let db_path = db_dir.join(network_name);