Sema: populate the sentinel for comptime array inits

test/behavior/array_llvm.zig

@@ -166,3 +166,16 @@ test "anonymous literal in array" {
     try S.doTheTest();
     comptime try S.doTheTest();
 }
+
+test "access the null element of a null terminated array" {
+    const S = struct {
+        fn doTheTest() !void {
+            var array: [4:0]u8 = .{ 'a', 'o', 'e', 'u' };
+            try expect(array[4] == 0);
+            var len: usize = 4;
+            try expect(array[len] == 0);
+        }
+    };
+    try S.doTheTest();
+    comptime try S.doTheTest();
+}