asn1, rrc: fix crash on s1 handover: allocate memory for source_other_cfg_r9 before unpacking data into it

[murzic4]

srseNB crashes when tries to unpack SEQUENCE AS-Config (RRC HandoverPreparationInformation message) with sourceOtherConfig-r9 field. It happens because RRC decoder tries to unpack sourceOtherConfig-r9 SEQUENCE to source_other_cfg_r9 which contains no preallocated memory.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}