fstests: log-writes: Handle unrecognized options to prevent segfault

[BUG]
When using --help parameter (unrecognized) after valid --log/--replay, log-writes just crashes:
Starting program: replay-log --log /dev/test/test --replay /dev/test/scratch1 --help /home/adam/xfstests-dev/src/log-writes/replay-log: unrecognized option '--help'

Program received signal SIGSEGV, Segmentation fault. 0x00007ffff7f5cc55 in __strlen_avx2 () from /usr/lib/libc.so.6 (gdb) bt

  #0  0x00007ffff7f5cc55 in __strlen_avx2 () from /usr/lib/libc.so.6
  #1  0x00007ffff7e89363 in strdup () from /usr/lib/libc.so.6
  #2  0x00005555555554ac in main (argc=6, argv=0x7fffffffea78)
      at replay-log.c:219

[CAUSE]
We didn't check return value from getopt_long() for unrecognized parameter, thus we reuse the old opt_index, and if that option needs an parameter, we will access optarg which can be NULL and cause segfault.

[FIX]
Check return value from getopt_long() for '?' to handle unrecognized options correctly.

---
 src/log-writes/replay-log.c | 2 ++
 1 file changed, 2 insertions(+)

index c16df40e..829b18e2 100644
@@ -162,6 +162,8 @@ int main(int argc, char **argv)
 		case 'v':
 			log_writes_verbose++;
 			continue;
+		case '?':
+			usage();
 		default:
 			break;
 		}

2.22.0