qemuBuildVideoCommandLine: Don't access def->videos without check

qemuBuildVideoCommandLine: Don't access def->videos without check

This function can be called over a domain definition that has no
video configured. The
tests/qemuxml2argvdata/qemuxml2argv-minimal.xml file could serve
as an example. Problem is, before the check that domain has some
or none video configured, def->videos is dereferenced causing a
segmentation fault in case there's none video configured.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>