Changes

mgw: osmux: Fix heap-use-after-free

As found by Asan on a osmo-mgw running in production:
"""
 ==2238035==ERROR: AddressSanitizer: heap-use-after-free on address 0x62100043bdca at pc 0x7f9bcebaa070 bp 0x7ffcb08f2150 sp 0x7ffcb08f2148
 READ of size 2 at 0x62100043bdca thread T0
     #0 0x7f9bcebaa06f in msgb_length src/core/msgb.c:287
     #1 0x55869457a8ff in conn_osmux_send_rtp src/libosmo-mgcp/mgcp_osmux.c:245
     #2 0x558694563a86 in mgcp_dispatch_rtp_bridge_cb src/libosmo-mgcp/mgcp_network.c:1347
     #3 0x5586945570a9 in rx_rtp src/libosmo-mgcp/mgcp_network.c:1550
     #4 0x5586945570a9 in rtp_recvfrom_cb src/libosmo-mgcp/mgcp_network.c:1505
     #5 0x7f9bcebc96cc in iofd_poll_ofd_cb_recvmsg_sendmsg src/core/osmo_io_poll.c:84
     #6 0x7f9bcebcb699 in iofd_poll_ofd_cb_dispatch src/core/osmo_io_poll.c:136
     #7 0x7f9bcebd7df5 in poll_disp_fds src/core/select.c:419
     #8 0x7f9bcebd7df5 in _osmo_select_main src/core/select.c:457
     #9 0x7f9bcebd8298 in osmo_select_main src/core/select.c:496
     #10 0x558694534f2e in main src/osmo-mgw/mgw_main.c:428
"""

Related: SYS#7450
Change-Id: Id90c77aaf44422c3ed70ffb06560537e920a468c