sdiff(1): Fix potential NULL deref in cleanup path

In the presence of the --diff-pid argument, it is possible for 'diffpipe' to be
NULL.  Only fclose() it if it was initialized.

Reported by:	Coverity
CID:		1355183
Sponsored by:	EMC / Isilon Storage Division

usr.bin/sdiff/sdiff.c

@@ -430,7 +430,8 @@ main(int argc, char **argv)
 		/* Close open files and pipe, delete temps */
 		fclose(file1);
 		fclose(file2);
-		fclose(diffpipe);
+		if (diffpipe != NULL)
+			fclose(diffpipe);
 		if (tmp1)
 			if (unlink(tmp1))
 				warn("Error deleting %s.", tmp1);