if in doubt, prefer unwrap_or_default()

if the past we had lots of crashes because of unexpected unwrap failures,
mostly related to string.
this commit avoids them eg. for string-conversions that may panic
eg. when encountering a null-byte or by logical programming errors
where an object is assumed to be set but is not under unexpected circumstances.

src/stock.rs

@@ -118,7 +118,7 @@ impl StockMessage {
     ///
     /// These could be used in logging calls, so no logging here.
     fn fallback(&self) -> &'static str {
-        self.get_str("fallback").unwrap()
+        self.get_str("fallback").unwrap_or_default()
     }
 }