(5.4) Fix crash when accepting early media after receiving an INVITE without SDP
Original master MR: !5373 (merged) jira: https://linphone.atlassian.net/browse/SDK-820
liblinphone 5.4: liblinphone!3731 (merged)
Original master MR: !5373 (merged) jira: https://linphone.atlassian.net/browse/SDK-820
liblinphone 5.4: liblinphone!3731 (merged)