fix overflow found while fuzzing

* allow file level `union {}` to parse as tuple field

this was found while fuzzing zls.

* before this patch the input `union {}` crashed the parser.  after
  this, it parses correctly just like `struct {}`.
* adds behavior tests for both inputs `struct {}` and `union {}`,
  checking that each becomes a file level tuple field.

1 files changed, 1 insertions, 0 deletions

diff --git a/test/behavior/empty_file_level_struct.zig b/test/behavior/empty_file_level_struct.zig
new file mode 100644
index 0000000000..86f0f2b3c7
--- /dev/null
+++ b/test/behavior/empty_file_level_struct.zig
@@ -0,0 +1 @@
+struct {}