stk: Fix crash when envelope returns sync

stk: Fix crash when envelope returns sync

ofonod[13066]: src/stk.c:stk_select_item()
ofonod[13066]: src/stk.c:stk_select_item()
ofonod[13066]: src/stk.c:stk_send_envelope()
ofonod[13066]: drivers/qmimodem/stk.c:qmi_envelope()
ofonod[13066]: src/stk.c:envelope_cb() length 0
ofonod[13066]: src/stk.c:menu_selection_envelope_cb()
ofonod[13066]: Sending Menu Selection to UICC failed
process 13066: arguments to dbus_message_new_error() were incorrect,
assertion "reply_to != NULL" failed in file dbus-message.c line 1333.
This is normally a bug in some application using the D-Bus library.
  D-Bus not built with -rdynamic so unable to print a backtrace
  ofonod[13066]: Aborting (signal 6) [./src/ofonod]
  ofonod[13066]: ++++++++ backtrace ++++++++