Check unpad computation

[ulope]

In

shutter/shlib/shcrypto/encryption.go

Lines 194 to 196 in 31707b7

	if paddingLength == 0 {
	return nil, errors.New("invalid padding length 0")
	}

a padding length of 0 is assumed to be invalid.
Check if this isn't a valid case that must be handled differently.