grub-fstest: fix descriptor leak

Found by: Coverity scan.

util/grub-fstest.c

@@ -570,12 +570,12 @@ argp_parser (int key, char *arg, struct argp_state *state)
 	    return 0;
 	  }
 	real_size = fread (buf, 1, 1024, f);
+	fclose (f);
 	if (real_size < 0)
 	  {
 	    printf (_("%s: error:"), program_name);
 	    printf (_("cannot read `%s': %s"), arg, strerror (errno));
 	    printf ("\n");
-	    fclose (f);
 	    return 0;
 	  }
 	grub_zfs_add_key (buf, real_size, 0);