Revert "Skip initializing optional arguments to nil" and add test

This reverts commit 1596853.
Expressions that run when the local is not given can observe the initial
state of the optional parameter locals.

Author: XrXr

bootstraptest/test_method.rb

@@ -1434,3 +1434,9 @@ def x = [1]
   def forwarder(...) = target(*x, 2, ...)
   forwarder(3).inspect
 }, '[Bug #21832] post-splat args before forwarding'
+
+assert_equal '[nil, nil]', %q{
+  def self_reading(a = a, kw:) = a
+  def through_binding(a = binding.local_variable_get(:a), kw:) = a
+  [self_reading(kw: 1), through_binding(kw: 1)]
+}, 'nil initialization of optional parameters'

vm_args.c

@@ -247,6 +247,7 @@ args_setup_opt_parameters(struct args_info *args, int opt_max, VALUE *locals)
         i = opt_max;
     }
     else {
+        int j;
         i = args->argc;
         args->argc = 0;
 
@@ -258,6 +259,11 @@ args_setup_opt_parameters(struct args_info *args, int opt_max, VALUE *locals)
                 locals[i] = argv[args->rest_index];
             }
         }
+
+        /* initialize by nil */
+        for (j=i; j<opt_max; j++) {
+            locals[j] = Qnil;
+        }
     }
 
     return i;