Add a check avoid segfault

If new_size was zero or realloc failed, it would be segment fault.
So add a check.

Signed-off-by: Jianpeng Ma <majianpeng@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/filesetup.c b/filesetup.c
index 69eb37c..ab5cc48 100644
--- a/filesetup.c
+++ b/filesetup.c
@@ -1079,6 +1079,10 @@
 		dprint(FD_FILE, "resize file array to %d files\n", new_size);
 
 		td->files = realloc(td->files, new_size * sizeof(f));
+		if (td->files == NULL) {
+			log_err("fio: realloc OOM\n");
+			assert(0);
+		}
 		td->files_size = new_size;
 	}
 	td->files[cur_files] = f;