Add a test for 'packet loss' decode calls

Tests 06643a44.

tests/fec.rs

@@ -0,0 +1,13 @@
+//! Test that supplying empty packets does forward error correction.
+
+extern crate opus;
+use opus::*;
+
+#[test]
+fn blah() {
+    let mut opus = Decoder::new(48000, Channels::Mono).unwrap();
+
+    let mut output = vec![0i16; 5760];
+    let size = opus.decode(&[], &mut output[..], true).unwrap();
+    assert_eq!(size, 5760);
+}