vmspawn: don't leak prior arg_directory in ephemeral branch

The ephemeral-snapshot branch did a bare arg_directory = strdup(...),
orphaning the -D/--directory allocation from parse_argv.
Use free_and_strdup(), matching the idiom already used elsewhere in
the file.

Co-developed-by: Claude Opus 4.8 <noreply@anthropic.com>
Signed-off-by: Paul Meyer <katexochen0@gmail.com>

src/vmspawn/vmspawn.c

@@ -3172,8 +3172,8 @@ static int run_virtual_machine(int kvm_device_fd, int vhost_device_fd) {
                         if (r < 0)
                                 return log_error_errno(r, "Failed to create ephemeral snapshot of '%s': %m", arg_directory);
 
-                        arg_directory = strdup(snapshot_directory);
-                        if (!arg_directory)
+                        r = free_and_strdup(&arg_directory, snapshot_directory);
+                        if (r < 0)
                                 return log_oom();
                 }