gprs_sndcp: fix use after free

When compression is turned on, an extra buffer "expnd" is allocated in
the context of msg. This means that when msg is freed, expnd is freed as
well and there is no need for freein it explcicitly, which, when it is
done after freeng msg, causes talloc to abort.

Change-Id: I8959b75e241ffabf9fa34c4cf014721584372b26