Fix BnString usage of as_ref instead of as_str

Fixes `BnString::len` and `BnString::is_empty` incorrectly reporting 1 and true on empty null terminated string.

Author: emesare

rust/src/string.rs

@@ -101,11 +101,11 @@ impl BnString {
     }
 
     pub fn len(&self) -> usize {
-        self.as_ref().len()
+        self.as_str().len()
     }
 
     pub fn is_empty(&self) -> bool {
-        self.as_ref().is_empty()
+        self.as_str().is_empty()
     }
 }