qemu-ga: fix segv after failure to open log file

qemu-ga: fix segv after failure to open log file

Currently, if we fail to open the specified log file (generally due to a
permissions issue), we'll assign NULL to the logfile handle (stderr,
initially) used by the logging routines, which can cause a segfault to
occur when we attempt to report the error before exiting.

Instead, only re-assign if the open() was successful.

Reviewed-by: Michal Privoznik <[email protected]>
Signed-off-by: Michael Roth <[email protected]>