Fix Coverity uninitialized pointer field Thread.h

Changed _obj_mem to be initialized with constructor of new thread.

Author: Tharazi97

rtos/source/Thread.cpp

@@ -51,7 +51,8 @@ void Thread::constructor(uint32_t tz_module, osPriority priority,
     const uint32_t offset = aligned_mem - unaligned_mem;
     const uint32_t aligned_size = ALIGN_DOWN(stack_size - offset, 8);
 
-    _tid = 0;
+    memset(&_obj_mem, 0, sizeof(_obj_mem));
+    _tid = nullptr;
     _dynamic_stack = (stack_mem == nullptr);
     _finished = false;
     memset(&_attr, 0, sizeof(_attr));
@@ -106,7 +107,6 @@ osStatus Thread::start(mbed::Callback<void()> task)
         ((uint32_t *)_attr.stack_mem)[i] = osRtxStackMagicWord;
     }
 
-    memset(&_obj_mem, 0, sizeof(_obj_mem));
     _attr.cb_size = sizeof(_obj_mem);
     _attr.cb_mem = &_obj_mem;
     _task = task;