std.zig: fix integer overflows during parsing

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()

lib/std/zig/parser_test.zig

@@ -221,6 +221,27 @@ test "zig fmt: top-level tuple function call type" {
     );
 }
 
+test "zig fmt: top-level enum missing 'const name ='" {
+    try testError(
+        \\enum(u32)
+        \\
+    , &[_]Error{.expected_token});
+}
+
+test "zig fmt: top-level bare asterisk+identifier" {
+    try testCanonical(
+        \\*x
+        \\
+    );
+}
+
+test "zig fmt: top-level bare asterisk+asterisk+identifier" {
+    try testCanonical(
+        \\**x
+        \\
+    );
+}
+
 test "zig fmt: C style containers" {
     try testError(
         \\struct Foo {