unpack-trees: handle lstat failure for existing directory

When check_leading_path notices no file in the way of the new entry to
be checked out, verify_absent checks whether there is a directory
there or nothing at all.  If that lstat call fails (for example due to
ENOMEM), it assumes ENOENT, meaning a directory with untracked files
would be clobbered in that case.

Check errno after calling lstat, and for conditions other than ENOENT,
just error out.

This is a theoretical race condition.  lstat has to succeed moments
before it fails for there to be trouble.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.c

@@ -1197,12 +1197,16 @@ static int verify_absent_1(struct cache_entry *ce,
 
 		return check_ok_to_remove(path, len, DT_UNKNOWN, NULL, &st,
 				error_type, o);
-	} else if (!lstat(ce->name, &st))
+	} else if (lstat(ce->name, &st)) {
+		if (errno != ENOENT)
+			return error("cannot stat '%s': %s", ce->name,
+				     strerror(errno));
+		return 0;
+	} else {
 		return check_ok_to_remove(ce->name, ce_namelen(ce),
-				ce_to_dtype(ce), ce, &st,
-				error_type, o);
-
-	return 0;
+					  ce_to_dtype(ce), ce, &st,
+					  error_type, o);
+	}
 }
 
 static int verify_absent(struct cache_entry *ce,