initialize previous_exception (#29861)

Fix uninitialized previous_exception bug introduced in #28878

Author: c42f

src/threading.c

@@ -283,6 +283,7 @@ static void ti_initthread(int16_t tid)
     memset(bt_data, 0, sizeof(uintptr_t) * (JL_MAX_BT_SIZE + 1));
     ptls->bt_data = (uintptr_t*)bt_data;
     ptls->sig_exception = NULL;
+    ptls->previous_exception = NULL;
 #ifdef _OS_WINDOWS_
     ptls->needs_resetstkoflw = 0;
 #endif