Avoid hanging on --jit-wait after MJIT.pause

When a worker is stopped, nobody will JIT a method for you.

mjit.c

@@ -328,6 +328,9 @@ mjit_wait(struct rb_iseq_constant_body *body)
 VALUE
 rb_mjit_wait_call(rb_execution_context_t *ec, struct rb_iseq_constant_body *body)
 {
+    if (worker_stopped)
+        return Qundef;
+
     mjit_wait(body);
     if ((uintptr_t)body->jit_func <= (uintptr_t)LAST_JIT_ISEQ_FUNC) {
         return Qundef;

mjit_worker.c

@@ -236,7 +236,7 @@ static int total_unloads = 0;
 // Set to true to stop worker.
 static bool stop_worker_p;
 // Set to true if worker is stopped.
-static bool worker_stopped;
+static bool worker_stopped = true;
 
 // Path of "/tmp", which can be changed to $TMP in MinGW.
 static char *tmp_dir;

test/ruby/test_jit.rb

@@ -1070,6 +1070,14 @@ class TestJIT < Test::Unit::TestCase
     end;
   end
 
+  def test_mjit_pause_wait
+    assert_eval_with_jit("#{<<~"begin;"}\n#{<<~"end;"}", stdout: '', success_count: 0, min_calls: 1)
+    begin;
+      RubyVM::MJIT.pause
+      proc {}.call
+    end;
+  end
+
   def test_caller_locations_without_catch_table
     out, _ = eval_with_jit("#{<<~"begin;"}\n#{<<~"end;"}", min_calls: 1)
     begin;