these were found while fuzzing zls.

this patch prevents overflow for the following file contents and adds
tests for them.
	* `enum(u32)` - causes overflow in std.zig.Ast.fullContainerDecl()
	* `*x` - causes overflow in std.zig.Ast.fullPtrType()
	* `**x` - causes overflow in std.zig.Ast.firstToken()