fix slice index out of range when dealing with compressed files.

Author: alexmullins

README.txt

@@ -67,5 +67,5 @@ Refer to http://www.winzip.com/aes_info.htm#winzip11 for the reasoning.
 16. Storage Format (file data payload) totals CompressedSize64 bytes:
     a. Salt - 8, 12, or 16 bytes depending on keysize
     b. Password Verification Value - 2 bytes
-    c. Encrypted Data - UncompressedSize64 bytes
+    c. Encrypted Data - compressed size - satl - pwv - auth lengths
     d. Authentication code - 10 bytes

reader.go

@@ -218,7 +218,7 @@ func newDecryptionReader(r io.Reader, f *File) (io.Reader, error) {
 	salt := content[:saltLen]
 	pwvv := content[saltLen : saltLen+2]
 	content = content[saltLen+2:]
-	size := f.UncompressedSize64
+	size := f.CompressedSize64 - uint64(saltLen) - 2 - 10
 	data := content[:size]
 	authcode := content[size:]
 	// generate keys