NR UE prevent segfault at detach

DCI formats 11 and 01 assume the presence of an active BWP, if not present return with an error to prevent segfault.

UE logs from CI:

610694.175746 [NAS] I [UE 0] Received NAS_DEREGISTRATION_REQ
610694.194338 [NAS] I [UE 0] Received NAS_DOWNLINK_DATA_IND: length 3 , buffer 0x6020000302d0
610694.194348 [NAS] I received deregistration accept
610694.400593 [NR_RRC] I [UE 0] Received RRC Release (gNB 0)
610694.400606 [PDCP] I release DRB 1 of UE 0
../../../openair2/LAYER2/NR_MAC_COMMON/nr_mac_common.c:3407:47: runtime error: member access within null pointer of type 'const struct NR_UE_DL_BWP_t'