crypt: Fix data corruption on seek

This was caused by failing to reset the internal buffer on seek so old
data was read first before the new data.

The unit tests didn't detect this because they were reading to the end
of the file to check integrity and thus emptying the internal buffer.

Both code and unit tests were fixed up.

crypt/cipher_test.go

@@ -881,15 +881,26 @@ func TestNewDecrypterSeek(t *testing.T) {
 		return ioutil.NopCloser(bytes.NewBuffer(ciphertext[int(underlyingOffset):])), nil
 	}
 
+	inBlock := make([]byte, 1024)
+
+	// Check the seek worked by reading a block and checking it
+	// against what it should be
+	check := func(rc ReadSeekCloser, offset int) {
+		n, err := io.ReadFull(rc, inBlock)
+		if err != nil && err != io.EOF && err != io.ErrUnexpectedEOF {
+			require.NoError(t, err)
+		}
+		seekedDecrypted := inBlock[:n]
+
+		require.Equal(t, plaintext[offset:offset+n], seekedDecrypted)
+	}
+
 	// Now try decoding it with a open/seek
 	for _, offset := range trials {
 		rc, err := c.DecryptDataSeek(open, int64(offset))
 		assert.NoError(t, err)
 
-		seekedDecrypted, err := ioutil.ReadAll(rc)
-		assert.NoError(t, err)
-
-		assert.Equal(t, plaintext[offset:], seekedDecrypted)
+		check(rc, offset)
 	}
 
 	// Now try decoding it with a single open and lots of seeks
@@ -898,10 +909,7 @@ func TestNewDecrypterSeek(t *testing.T) {
 		_, err := rc.Seek(int64(offset), 0)
 		assert.NoError(t, err)
 
-		seekedDecrypted, err := ioutil.ReadAll(rc)
-		assert.NoError(t, err)
-
-		assert.Equal(t, plaintext[offset:], seekedDecrypted)
+		check(rc, offset)
 	}
 }