fix:Race condition on instance variable assignment

lib/ddtrace/workers/async.rb

@@ -42,7 +42,7 @@ def terminate
         end
 
         def run_async?
-          @run_async = false unless instance_variable_defined?(:@run_async)
+          return false unless instance_variable_defined?(:@run_async)
           @run_async == true
         end
 
@@ -55,7 +55,7 @@ def running?
         end
 
         def error?
-          @error = nil unless instance_variable_defined?(:@error)
+          return false unless instance_variable_defined?(:@error)
           !@error.nil?
         end
 

lib/ddtrace/workers/loop.rb

@@ -33,7 +33,7 @@ def work_pending?
       end
 
       def run_loop?
-        @run_loop = false unless instance_variable_defined?(:@run_loop)
+        return false unless instance_variable_defined?(:@run_loop)
         @run_loop == true
       end
 

lib/ddtrace/workers/polling.rb

@@ -34,7 +34,7 @@ def stop(force_stop = false, timeout = SHUTDOWN_TIMEOUT)
       end
 
       def enabled?
-        @enabled = true unless instance_variable_defined?(:@enabled)
+        return true unless instance_variable_defined?(:@enabled)
         @enabled
       end