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 <mprivozn@redhat.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>

qemu-ga.c

@@ -836,12 +836,13 @@ int main(int argc, char **argv)
             become_daemon(pid_filepath);
         }
         if (log_filepath) {
-            s->log_file = fopen(log_filepath, "a");
-            if (!s->log_file) {
+            FILE *log_file = fopen(log_filepath, "a");
+            if (!log_file) {
                 g_critical("unable to open specified log file: %s",
                            strerror(errno));
                 goto out_bad;
             }
+            s->log_file = log_file;
         }
     }