This patch fixes a bug in the original driver: write to NULL descriptor.

b/182179434

--- a/src/wrapper.c
+++ b/src/wrapper.c
@@ -244,6 +244,9 @@ main (int argc, char *argv[])
 				break;
 			}
 
+			if (pfp == NULL) // do not write anything if pipe is not open
+				break;
+
 			if(first_fwrite){//最初のfwriteだけ、ページ数を送信
 				fwrite (&pageNum, 1, 1, pfp);
 				first_fwrite = FALSE;