mtest: Check that stdout is not None before seeking
mtest: Check that stdout is not None before seeking

As can be the case if additional_error is None and verbose is used.

Fixes #4470

QuLogic/meson,MathieuDuponchelle/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,MathieuDuponchelle/meson,pexip/meson,pexip/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,MathieuDuponchelle/meson,QuLogic/meson,jpakkane/meson,becm/meson,QuLogic/meson,becm/meson,QuLogic/meson,becm/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,MathieuDuponchelle/meson,becm/meson,jpakkane/meson,MathieuDuponchelle/meson,becm/meson,pexip/meson,QuLogic/meson,becm/meson,MathieuDuponchelle/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,MathieuDuponchelle/meson,mesonbuild/meson,pexip/meson,becm/meson,pexip/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,MathieuDuponchelle/meson,becm/meson,mesonbuild/meson,jpakkane/meson,becm/meson,jpakkane/meson,pexip/meson,MathieuDuponchelle/meson,pexip/meson,becm/meson,mesonbuild/meson