CLI: Ensure IDs are non-zero

This fixes out-of-bounds accesses in several subcommands when the
invalid ID @0 is passed to the program.

src/CLI.cpp

@@ -412,7 +412,8 @@ void CLI::identifyIds ()
       int digits;
       if (pig.skipLiteral ("@")  &&
           pig.getDigits (digits) &&
-          pig.eos ())
+          pig.eos ()             &&
+          digits > 0)
       {
         a.tag ("ID");
         a.attribute ("value", digits);

test/ids.t

@@ -48,6 +48,10 @@ class TestIds(TestCase):
         self.assertIn(' @1 ', out)
         self.assertIn(' @2 ', out)
 
+    def test_should_fail_on_zero_id(self):
+        code, out, err = self.t.runError("delete @0")
+        self.assertIn("IDs must be specified.", err)
+
 
 if __name__ == "__main__":
     from simpletap import TAPTestRunner