Fix cross-interpreter NotShareableError ref handling

Author: lpyu001

Python/crossinterp_data_lookup.h

@@ -100,6 +100,7 @@ _PyXI_UnwrapNotShareableError(PyThreadState * tstate, _PyXI_failure *failure)
     if (failure != NULL) {
         _PyXI_errcode code = _PyXI_ERR_NOT_SHAREABLE;
         if (_PyXI_InitFailure(failure, code, exc) < 0) {
+            Py_DECREF(exc);
             return -1;
         }
     }

Python/crossinterp_exceptions.h

@@ -83,6 +83,7 @@ _ensure_notshareableerror(PyThreadState *tstate,
             // A NotShareableError instance is already set.
             assert(cause == NULL);
             _PyErr_SetRaisedException(tstate, ctx);
+            return;
         }
     }
     else {