read-tree: fix bogus debugging statement.

We wanted to detect case #16 which should be rare, but botched the
case when some paths are missing, causing a segfault.  My fault.

Signed-off-by: Junio C Hamano <junkio@cox.net>

read-tree.c

@@ -362,12 +362,15 @@ static int keep_entry(struct cache_entry *ce)
 static void show_stage_entry(FILE *o,
 			     const char *label, const struct cache_entry *ce)
 {
-	fprintf(stderr, "%s%06o %s %d\t%s\n",
-		label,
-		ntohl(ce->ce_mode),
-		sha1_to_hex(ce->sha1),
-		ce_stage(ce),
-		ce->name);
+	if (!ce)
+		fprintf(o, "%s (missing)\n", label);
+	else
+		fprintf(o, "%s%06o %s %d\t%s\n",
+			label,
+			ntohl(ce->ce_mode),
+			sha1_to_hex(ce->sha1),
+			ce_stage(ce),
+			ce->name);
 }
 #endif