SMACK-450 Make VCard.doLoad() return if the received packet was not an error and also not an VCard. Avoid calling VCard.load() with 'null' as argument.

git-svn-id: http://svn.igniterealtime.org/svn/repos/smack/branches/smack_3_3_1@13713 b35dd754-fafc-0310-a699-88a17e54d16e