py/runtime: Initialize profile fields in mp_thread_init_state.

If the fields added for `MICROPY_PY_SYS_SETTRACE` are not initialized
properly, their value in a thread is indeterminate.  In particular, if the
callback is not NULL, it will be invoked as a function.

Signed-off-by: Jeff Epler <jepler@gmail.com>

Author: jepler

py/runtime.h

@@ -169,6 +169,12 @@ static inline void mp_thread_init_state(mp_state_thread_t *ts, size_t stack_size
     ts->nlr_jump_callback_top = NULL;
     ts->mp_pending_exception = MP_OBJ_NULL;
 
+    #if MICROPY_PY_SYS_SETTRACE
+    ts->prof_trace_callback = MP_OBJ_NULL;
+    ts->prof_callback_is_executing = false;
+    ts->current_code_state = NULL;
+    #endif
+
     // If locals/globals are not given, inherit from main thread
     if (locals == NULL) {
         locals = mp_state_ctx.thread.dict_locals;