Changes

gprs_ms_net_cap_gea_mask(): protect against empty MS Network Capabilities

When the MS Network Capability is empty, expect to support at least
unencrypted communications.
This shouldn't be empty at all, as the minimum length of it is 2 when
the MS/UE is doing an Attach Request.
But when receiving a MS/UE SGSN Context via Gn, the field
is optional.
The MS/UE shall include it into the Routing Area Request,
but not must.

Change-Id: Ieef2e3eeaaadc90c35fff6f20d47bd36aaa4b9e6
(cherry picked from commit d3b037ccfae542807b7602c2163f11fa0eb3b885)