Fix YangSchema::enableFeature segfault on invalid modules

Now throws an exception on invalid modules.

Change-Id: I494813fa5bc3f8357d98e731f7b2748e2df3ee90
diff --git a/tests/yang.cpp b/tests/yang.cpp
index f50d76e..833ef70 100644
--- a/tests/yang.cpp
+++ b/tests/yang.cpp
@@ -1183,5 +1183,10 @@
 
             REQUIRE_THROWS(ys.nodeType(path, node));
         }
+
+        SECTION("enableFeature - non existing module")
+        {
+            REQUIRE_THROWS_AS(ys.enableFeature("non-existing", "just-no"), std::runtime_error);
+        }
     }
 }