The existing check for the initialization of GTP is wrong; GTP returns
-1 if it could not initialize. Correct this, such that the DU will stop
if GTP cannot be initialize.

If we continue, the DU is requested to set up a GTP tunnel; because GTP
is not initialized, it assumes it is in monolithic/no F1, and does not
set up a tunnel. The F1 UE Context setup response encoder then fails
with this assertion:

    Assertion (tnl->teid > 0) failed!
    In DU_send_UE_CONTEXT_SETUP_RESPONSE() ../../../openair2/F1AP/f1ap_du_ue_context_management.c:415

    Exiting execution
    ../../../openair2/F1AP/f1ap_du_ue_context_management.c:415 DU_send_UE_CONTEXT_SETUP_RESPONSE() Exiting OAI softmodem: _Assert_Exit_
    Aborted

Closes: #853