gprs_llc: tx dl ud: make mismatching LLE not crash osmo-sgsn

On 34c3, osmo-sgsn keeps restarting. At least once, it hits the assertion that
this patch replaces with an error message, to not disrupt operation.

Change-Id: I07a40960920dbc594192530c3a145f9a5d2a9c81
1 file changed