Fix next inside block argument stack underflow

[Bug #20344]
Fix compile_next adding removable adjust label

compile.c

@@ -3070,8 +3070,7 @@ remove_unreachable_chunk(rb_iseq_t *iseq, LINK_ELEMENT *i)
             /* do nothing */
         }
         else if (IS_ADJUST(i)) {
-            LABEL *dest = ((ADJUST *)i)->label;
-            if (dest && dest->unremovable) return 0;
+            return 0;
         }
         end = i;
     } while ((i = i->next) != 0);
@@ -8205,7 +8204,6 @@ compile_next(rb_iseq_t *iseq, LINK_ANCHOR *const ret, const NODE *const node, in
         add_ensure_iseq(ret, iseq, 0);
         ADD_INSNL(ret, line_node, jump, ISEQ_COMPILE_DATA(iseq)->end_label);
         ADD_ADJUST_RESTORE(ret, splabel);
-        splabel->unremovable = FALSE;
 
         if (!popped) {
             ADD_INSN(ret, line_node, putnil);

test/ruby/test_iseq.rb

@@ -828,6 +828,27 @@ class TestISeq < Test::Unit::TestCase
     end;
   end
 
+  def test_unreachable_next_in_block
+    bug20344 = '[ruby-core:117210] [Bug #20344]'
+    assert_nothing_raised(SyntaxError, bug20344) do
+      compile(<<~RUBY)
+        proc do
+          next
+
+          case nil
+          when "a"
+            next
+          when "b"
+          when "c"
+            proc {}
+          end
+
+          next
+        end
+      RUBY
+    end
+  end
+
   def test_loading_kwargs_memory_leak
     assert_no_memory_leak([], "#{<<~"begin;"}", "#{<<~'end;'}", rss: true)
     a = RubyVM::InstructionSequence.compile("foo(bar: :baz)").to_binary