Fix match bug when no case matches (#677)

executable_semantics/testdata/no_match.carbon

@@ -0,0 +1,12 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+fn main() -> Int {
+  var x: Int = 0;
+  match (x) {
+    case 1 =>
+      return 1;
+  }
+  return 0;
+}

executable_semantics/testdata/no_match.golden

@@ -0,0 +1 @@
+result: 0