astgen: fix cppcheck warnings for err_scope_used and param_insts

Use memset initialization to satisfy cppcheck's data flow analysis:
- err_scope_used: cppcheck can't track writes through is_used_or_discarded pointer
- param_insts: cppcheck warns about potentially uninitialized array elements

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/std/zig/parser_test.zig

@@ -7,7 +7,9 @@ const maxInt = std.math.maxInt;
 const zig0 = if (@hasDecl(@import("root"), "zig0"))
     @import("root").zig0
 else
-    struct { pub const enabled = false; };
+    struct {
+        pub const enabled = false;
+    };
 
 test "zig fmt: remove extra whitespace at start and end of file with comment between" {
     try testTransform(