mixin: fix a NULL dereference

[lyakh]

In mixout_prepare() mixin->pipeline can be NULL, check it before dereferencing.

Fixes the second part of #6118

[lgirdwood]

Rerun CI - license server slows us down again.

License checkout failed: No such feature exists.
Feature:       XTENSA_XCC_TIE
License path:  84300@xtensa01p.elic.intel.com:/srv/home/jenkins/xcc/install/tools/RG-2017.8-linux/XtensaTools/Tools/lic/license.dat:
FLEXnet Licensing error:-5,147
For further information, refer to the FLEXnet Licensing documentation,
available at "www.macrovision.com".
make[3]: *** [CMakeFiles/sof.dir/build.make:258: CMakeFiles/sof.dir/src/arch/xtensa/lib/cpu.c.o] Error 2

[lgirdwood]

SOFCI TEST

[lyakh]

CI failures are unrelated because this PR only touches IPC4 code

[lgirdwood]

@wszypelt @lrudyX Could you check CI, not expecting this PR to fail those tests. Thanks

[wszypelt]

@lgirdwood I ran tests on CI again