posix: fix race condition between pthread_create and capture engine

Fix PR 2068:
Reproducable crashes occur when using pthreads and the capture engine
at the same time. 'pthread_create()' is the culprit. It creates a SCORE thread
and then calls Thread_Start( ) without disabling thread-dispatching.

cpukit/posix/src/pthreadcreate.c

@@ -198,6 +198,8 @@ int pthread_create(
    */
   the_thread->do_post_task_switch_extension = true;
 
+  _Thread_Disable_dispatch();
+
   /*
    *  POSIX threads are allocated and started in one operation.
    */
@@ -217,6 +219,7 @@ int pthread_create(
      *        thread while we are creating it.
      */
     if ( !status ) {
+      _Thread_Enable_dispatch();
       _POSIX_Threads_Free( the_thread );
       _RTEMS_Unlock_allocator();
       return EINVAL;
@@ -230,6 +233,8 @@ int pthread_create(
     );
   }
 
+  _Thread_Enable_dispatch();
+
   /*
    *  Return the id and indicate we successfully created the thread
    */