parser: ostensibly fix sentinel handling

I guess arrays don't need special handling because their memory is
explicitly accounted for, but it would probably be good to check that
a sentinel-terminated array initialized as `undefined` does get the
correct sentinel value.

examples/reify.zig

@@ -9,7 +9,7 @@ const Example = struct {
     useful: bool,
     number: i32,
     string: []const u8,
-    longstring: []const u8,
+    longstring: [:0]const u8,
     tuple: struct { bool, i8 },
     enume: enum { first, second, third },
     taggart: union(enum) { first: []const u8, second: i32 },