Fix potential NULL dereference in alloc_mem_mmap()

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/memory.c b/memory.c
index b208320..8c06d94 100644
--- a/memory.c
+++ b/memory.c
@@ -158,9 +158,10 @@
 	if (td->orig_buffer == MAP_FAILED) {
 		td_verror(td, errno, "mmap");
 		td->orig_buffer = NULL;
-		if (td->mmapfd) {
+		if (td->mmapfd != 1) {
 			close(td->mmapfd);
-			unlink(td->o.mmapfile);
+			if (td->o.mmapfile)
+				unlink(td->o.mmapfile);
 		}
 
 		return 1;