Fix double free issue in qemu_set_log_filename().

Fix double free issue in qemu_set_log_filename().

After freeing the logfilename, we set logfilename to NULL, in case of an
error which returns without setting logfilename.

Signed-off-by: Robert Foley <robert.foley@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20191118211528.3221-2-robert.foley@linaro.org>
(cherry picked from commit 0f516ca4767042aec8716369d6d62436fa10593a)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>