qemu: Resolve Coverity FORWARD_NULL
qemu: Resolve Coverity FORWARD_NULL

If the qemuMigrationEatCookie() fails to set mig, we jump to cleanup:
which will call qemuMigrationCancelDriveMirror() without first checking
if mig == NULL

Signed-off-by: John Ferlan <87558058f6f829e5ec976c8ef960720af4ff9c7d@redhat.com>

jardasgit/libvirt,taget/libvirt,nertpinx/libvirt,rlaager/libvirt,VenkatDatta/libvirt,elmarco/libvirt,jfehlig/libvirt,crobinso/libvirt,rlaager/libvirt,elmarco/libvirt,agx/libvirt,taget/libvirt,eskultety/libvirt,zippy2/libvirt,shugaoye/libvirt,zippy2/libvirt,datto/libvirt,shugaoye/libvirt,eskultety/libvirt,libvirt/libvirt,olafhering/libvirt,agx/libvirt,datto/libvirt,elmarco/libvirt,datto/libvirt,eskultety/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,shugaoye/libvirt,VenkatDatta/libvirt,nertpinx/libvirt,rlaager/libvirt,eskultety/libvirt,rlaager/libvirt,olafhering/libvirt,agx/libvirt,olafhering/libvirt,nertpinx/libvirt,zippy2/libvirt,crobinso/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jfehlig/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,jardasgit/libvirt,elmarco/libvirt,andreabolognani/libvirt,elmarco/libvirt,shugaoye/libvirt,VenkatDatta/libvirt,datto/libvirt,nertpinx/libvirt,jardasgit/libvirt,eskultety/libvirt,agx/libvirt,fabianfreyer/libvirt,libvirt/libvirt,crobinso/libvirt,libvirt/libvirt,zippy2/libvirt,datto/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,taget/libvirt,VenkatDatta/libvirt,andreabolognani/libvirt,andreabolognani/libvirt,nertpinx/libvirt,taget/libvirt,jfehlig/libvirt,jfehlig/libvirt,crobinso/libvirt,fabianfreyer/libvirt,shugaoye/libvirt,libvirt/libvirt,rlaager/libvirt,olafhering/libvirt,taget/libvirt,agx/libvirt