Fix error handling when thread cannot be created

Update the Thread::start function to gracefully handle the failed
creation of a thread when there are no TCBs left. This patch does
the following:
1. Set memory handles to NULL after free to prevent double free
2. Post to the release semaphore so anything that tries to join this
    thread will join immediately
3. Remove dead return path since the new operator should never
    return NULL (it should trap instead)

rtos/rtos/Thread.cpp

@@ -83,10 +83,7 @@ osStatus Thread::start(Callback<void()> task) {
     _thread_def.pthread = Thread::_thunk;
     if (_thread_def.stack_pointer == NULL) {
         _thread_def.stack_pointer = new uint32_t[_thread_def.stacksize/sizeof(uint32_t)];
-        if (_thread_def.stack_pointer == NULL) {
-            _mutex.unlock();
-            return osErrorNoMemory;
-        }
+        MBED_ASSERT(_thread_def.stack_pointer != NULL);
     }
 
     //Fill the stack with a magic word for maximum usage checking
@@ -97,8 +94,12 @@ osStatus Thread::start(Callback<void()> task) {
     _task = task;
     _tid = osThreadCreate(&_thread_def, this);
     if (_tid == NULL) {
-        if (_dynamic_stack) delete[] (_thread_def.stack_pointer);
+        if (_dynamic_stack) {
+            delete[] (_thread_def.stack_pointer);
+            _thread_def.stack_pointer = (uint32_t*)NULL;
+        }
         _mutex.unlock();
+        _join_sem.release();
         return osErrorResource;
     }
 
@@ -355,6 +356,7 @@ Thread::~Thread() {
 #ifdef __MBED_CMSIS_RTOS_CM
     if (_dynamic_stack) {
         delete[] (_thread_def.stack_pointer);
+        _thread_def.stack_pointer = (uint32_t*)NULL;
     }
 #endif
 }