in case length is zero. Also do throw a NegativeArraySizeException if
length is negative instead of returning null.
This fixes the following sporadic test issue:
org.jivesoftware.smackx.ox.PainlessOpenPgpProviderTest > encryptDecryptTest FAILED
java.lang.AssertionError
at org.jivesoftware.smack.util.XmlStringBuilder.escape(XmlStringBuilder.java:425)
at org.jivesoftware.smackx.ox.element.EncryptedOpenPgpContentElement.addCommonXml(EncryptedOpenPgpContentElement.java:65)
at org.jivesoftware.smackx.ox.element.CryptElement.toXML(CryptElement.java:51)
at org.jivesoftware.smackx.ox.element.CryptElement.toXML(CryptElement.java:31)
at org.jivesoftware.smack.packet.Element.toXML(Element.java:41)
at org.jivesoftware.smackx.ox.element.OpenPgpContentElement.toInputStream(OpenPgpContentElement.java:186)
at org.jivesoftware.smackx.ox.crypto.PainlessOpenPgpProvider.encrypt(PainlessOpenPgpProvider.java:136)
at org.jivesoftware.smackx.ox.PainlessOpenPgpProviderTest.encryptDecryptTest(PainlessOpenPgpProviderTest.java:155)
because EncryptedOpenPgpContentElement rpad field was sometimes
'null' in case the random function returned '0' as length.