Merge branch '1433-illegal-characters' into 'master'

Handle illegal byte sequences safely in BdfReaderImpl

Closes #1433

See merge request briar/briar!967

bramble-api/src/main/java/org/briarproject/bramble/util/StringUtils.java

@@ -47,7 +47,7 @@ public class StringUtils {
 		try {
 			return s.getBytes("UTF-8");
 		} catch (UnsupportedEncodingException e) {
-			throw new RuntimeException(e);
+			throw new AssertionError(e);
 		}
 	}
 
@@ -63,7 +63,7 @@ public class StringUtils {
 		try {
 			return decoder.decode(buffer).toString();
 		} catch (CharacterCodingException e) {
-			throw new RuntimeException(e);
+			throw new AssertionError(e);
 		}
 	}