Check the gsm0503_xcch_decode return value

- Discard the message when gsm0503_xcch_decode returns -1
- Add automated decrypt test

See https://groups.google.com/d/msg/gr-gsm/9nOkIdrGOck/qTZh47McCQAJ
diff --git a/.travis.yml b/.travis.yml
index fde90b4..2d66445 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -20,3 +20,4 @@
 script:
 #  - docker run -it --rm $IMGNAME sh -c 'cd /src/build;make test'
   - docker run -it --rm $IMGNAME /src/tests/scripts/decode.sh
+  - docker run -it --rm $IMGNAME /src/tests/scripts/decrypt.sh