libqtest: check for g_setenv() failure

libqtest: check for g_setenv() failure

g_setenv() can fail; check for it when starting a QEMU process
when we set the QEMU_AUDIO_DRV environment variable.

Because this happens after fork() reporting an exact message
via printf() is a bad idea; just exit(1), as we already do
for the case of execlp() failure.

Fixes: Coverity CID 1460117
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20210820163750.9106-1-peter.maydell@linaro.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>