Setting environment variables can fail; check the return value
from g_setenv() and bail out if we couldn't set SDL_VIDEODRIVER.

Fixes: Coverity 1458798
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20210809161424.32355-1-peter.maydell@linaro.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>