migration/rdma: fix a memleak on error path in rdma_start_incoming_migration

migration/rdma: fix a memleak on error path in rdma_start_incoming_migration

'rdma->host' is malloced in qemu_rdma_data_init, but forgot to free on the error
path in rdma_start_incoming_migration(), this patch fix that.

The leak stack:
Direct leak of 2 byte(s) in 1 object(s) allocated from:
    #0 0x7fb7add18ae8 in __interceptor_malloc (/lib64/libasan.so.5+0xefae8)
    #1 0x7fb7ad0df1d5 in g_malloc (/lib64/libglib-2.0.so.0+0x531d5)
    #2 0x7fb7ad0f8b32 in g_strdup (/lib64/libglib-2.0.so.0+0x6cb32)
    #3 0x55a0464a0f6f in qemu_rdma_data_init /mnt/sdb/qemu/migration/rdma.c:2647
    #4 0x55a0464b0e76 in rdma_start_incoming_migration /mnt/sdb/qemu/migration/rdma.c:4020
    #5 0x55a0463f898a in qemu_start_incoming_migration /mnt/sdb/qemu/migration/migration.c:365
    #6 0x55a0458c75d3 in qemu_init /mnt/sdb/qemu/softmmu/vl.c:4438
    #7 0x55a046a3d811 in main /mnt/sdb/qemu/softmmu/main.c:48
    #8 0x7fb7a8417872 in __libc_start_main (/lib64/libc.so.6+0x23872)
    #9 0x55a04536b26d in _start (/mnt/sdb/qemu/build/x86_64-softmmu/qemu-system-x86_64+0x286926d)

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Pan Nengyuan <pannengyuan@huawei.com>
Message-Id: <20200420102727.17339-1-pannengyuan@huawei.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>