seekable zip backend: Accept files with valid ZIP64 EOCD headers

Previously, a ZIP64 file needed to have a valid classic EOCD header.
This is not the case at least for archives generated by PHPZipStreamer.

This commit allows for a bit more than necessary: While PHPZipStreamer
sets the classic EOCD header to all -1's, the bidding function now
succeeds also if the EOCD header values have arbitrary values, as long
as the ZIP64 EOCD header seems plausible. This simplifies the logic and
should not do any harm in practice.

This fixes #869.

libarchive/archive_read_support_format_zip.c

@@ -2407,7 +2407,7 @@ read_eocd(struct zip *zip, const char *p, int64_t current_offset)
  * Examine Zip64 EOCD locator:  If it's valid, store the information
  * from it.
  */
-static void
+static int
 read_zip64_eocd(struct archive_read *a, struct zip *zip, const char *p)
 {
 	int64_t eocd64_offset;
@@ -2417,35 +2417,37 @@ read_zip64_eocd(struct archive_read *a, struct zip *zip, const char *p)
 
 	/* Central dir must be on first volume. */
 	if (archive_le32dec(p + 4) != 0)
-		return;
+		return 0;
 	/* Must be only a single volume. */
 	if (archive_le32dec(p + 16) != 1)
-		return;
+		return 0;
 
 	/* Find the Zip64 EOCD record. */
 	eocd64_offset = archive_le64dec(p + 8);
 	if (__archive_read_seek(a, eocd64_offset, SEEK_SET) < 0)
-		return;
+		return 0;
 	if ((p = __archive_read_ahead(a, 56, NULL)) == NULL)
-		return;
+		return 0;
 	/* Make sure we can read all of it. */
 	eocd64_size = archive_le64dec(p + 4) + 12;
 	if (eocd64_size < 56 || eocd64_size > 16384)
-		return;
+		return 0;
 	if ((p = __archive_read_ahead(a, (size_t)eocd64_size, NULL)) == NULL)
-		return;
+		return 0;
 
 	/* Sanity-check the EOCD64 */
 	if (archive_le32dec(p + 16) != 0) /* Must be disk #0 */
-		return;
+		return 0;
 	if (archive_le32dec(p + 20) != 0) /* CD must be on disk #0 */
-		return;
+		return 0;
 	/* CD can't be split. */
 	if (archive_le64dec(p + 24) != archive_le64dec(p + 32))
-		return;
+		return 0;
 
 	/* Save the central directory offset for later use. */
 	zip->central_directory_offset = archive_le64dec(p + 48);
+
+	return 32;
 }
 
 static int
@@ -2483,15 +2485,14 @@ archive_read_format_zip_seekable_bid(struct archive_read *a, int best_bid)
 			if (memcmp(p + i, "PK\005\006", 4) == 0) {
 				int ret = read_eocd(zip, p + i,
 				    current_offset + i);
-				if (ret > 0) {
-					/* Zip64 EOCD locator precedes
-					 * regular EOCD if present. */
-					if (i >= 20
-					    && memcmp(p + i - 20, "PK\006\007", 4) == 0) {
-						read_zip64_eocd(a, zip, p + i - 20);
-					}
-					return (ret);
+				/* Zip64 EOCD locator precedes
+				 * regular EOCD if present. */
+				if (i >= 20 && memcmp(p + i - 20, "PK\006\007", 4) == 0) {
+					int ret_zip64 = read_zip64_eocd(a, zip, p + i - 20);
+					if (ret_zip64 > ret)
+						ret = ret_zip64;
 				}
+				return (ret);
 			}
 			i -= 4;
 			break;