Copy the m_mode before freeing the parser.
Address sanitizer aborted with:
20210601033017695 DSIP INFO re-INVITE for call 854A5CDA8037073 (sip.c:192)
=================================================================
==8583==ERROR: AddressSanitizer: heap-use-after-free on address 0x612000003250 at pc 0x55c3b4624dc5 bp 0x7ffe8a4464d0 sp 0x7ffe8a4464c8
READ of size 8 at 0x612000003250 thread T0
#0 0x55c3b4624dc4 in sdp_get_sdp_mode ../../../src/osmo-sip-connector/src/sdp.c:72
#1 0x55c3b462be9e in sip_handle_reinvite ../../../src/osmo-sip-connector/src/sip.c:202
#2 0x55c3b462d676 in nua_callback ../../../src/osmo-sip-connector/src/sip.c:397
[...]
Change-Id: I4c48832f01e61e98536de8f164ab5a3caa64f34a