sasl: Fix crash when ending a SASL session
sasl: Fix crash when ending a SASL session

When exiting remote-viewer after authenticating through SASL, I got
this crash:

 #0  0x0000000100a51870 in ?? ()
 #1  0x000000314d20c53e in _sasl_log (conn=<optimized out>, level=5, fmt=0x7fffe49893e8 "DIGEST-MD5 client mech dispose")
     at common.c:1985
 #2  0x00007fffe4982d88 in digestmd5_client_mech_dispose (conn_context=0xaf1900, utils=0xaefd10) at digestmd5.c:4580
 #3  0x000000314d208654 in client_dispose (pconn=0xaf0710) at client.c:332
 #4  0x000000314d20b76b in sasl_dispose (pconn=0xa51898) at common.c:851
 #5  0x00007ffff7602dc7 in channel_reset (channel=0xa52250, migrating=0) at spice-channel.c:2493
 #6  0x00007ffff760f7b7 in spice_inputs_channel_reset (channel=0xa52250, migrating=0) at channel-inputs.c:615
 #7  0x00007ffff76030ac in spice_channel_reset (channel=0xa52250, migrating=0) at spice-channel.c:2551
 #8  0x00007ffff76031e0 in channel_disconnect (channel=0xa52250) at spice-channel.c:2570
 #9  0x00007ffff760283d in spice_channel_coroutine (data=0xa52250) at spice-channel.c:2368
 #10 0x00007ffff763d14b in coroutine_trampoline (cc=0xa51900) at coroutine_ucontext.c:58
 #11 0x00007ffff763ce30 in continuation_trampoline (i0=10819840, i1=0) at continuation.c:49
 #12 0x00000031342479c0 in ?? () from /lib64/libc.so.6
 #13 0x0000000000a51cc8 in ?? ()
 #14 0x0000000000000000 in ?? ()

It turns out that the sasl_callback_t data passed when calling
sasl_client_new() must be valid until sasl_dispose() is called. I could not
find mentions of this in the official documentation but
https:cb3cb70173d8e81289fc157792a252d701947323@ted.stsp.name%3E
describes what happens.
Making the sasl_callback_t structure static should be enough to guarantee
that the data will stay around long enough.

elmarco/spice-gtk,flexVDI/spice-gtk,elmarco/spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,flexVDI/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,guodong/spice-gtk,fgouget/spice-gtk