Backport zip64 extra field improvements from Go's stdlib.

This fix allows zip to read archives that include files larger than 4GB
and/or those that use zip64 extra fields. This is simply a backport of
fixes that were made in http://golang.org/issue/13367.

Author: mikewiacek

reader.go

@@ -281,6 +281,10 @@ func readDirectoryHeader(f *File, r io.Reader) error {
 	f.Extra = d[filenameLen : filenameLen+extraLen]
 	f.Comment = string(d[filenameLen+extraLen:])
 
+	needUSize := f.UncompressedSize == ^uint32(0)
+	needCSize := f.CompressedSize == ^uint32(0)
+	needHeaderOffset := f.headerOffset == int64(^uint32(0))
+
 	if len(f.Extra) > 0 {
 		b := readBuf(f.Extra)
 		for len(b) >= 4 { // need at least tag and size
@@ -290,16 +294,32 @@ func readDirectoryHeader(f *File, r io.Reader) error {
 				return ErrFormat
 			}
 			eb := readBuf(b[:size])
+
 			switch tag {
 			case zip64ExtraId:
-				// update directory values from the zip64 extra block
-				if len(eb) >= 8 {
+				// update directory values from the zip64 extra block.
+				// They should only be consulted if the sizes read earlier
+				// are maxed out.
+				// See golang.org/issue/13367.
+				if needUSize {
+					needUSize = false
+					if len(eb) < 8 {
+						return ErrFormat
+					}
 					f.UncompressedSize64 = eb.uint64()
 				}
-				if len(eb) >= 8 {
+				if needCSize {
+					needCSize = false
+					if len(eb) < 8 {
+						return ErrFormat
+					}
 					f.CompressedSize64 = eb.uint64()
 				}
-				if len(eb) >= 8 {
+				if needHeaderOffset {
+					needHeaderOffset = false
+					if len(eb) < 8 {
+						return ErrFormat
+					}
 					f.headerOffset = int64(eb.uint64())
 				}
 			case winzipAesExtraId: