process_ms_ctx_status: Fix crash deleting PDP Ctx if GTP side was already released

sgsn_delete_pdp_ctx() should never be called without checking if the GTP
side is available, since it may happen that it has already been released
by the time the mmctx tells us the pdp ctx is gone on the MS side.

Fixes: OS#4817
Change-Id: Ie618874545172ec98355174a2ee041fc4a8bec16