Fixed crash in stanza_create_form when text is NULL

xmpp_stanza_get_text may return NULL.
Also fixed memory leak: xmpp_stanza_get_text returns new allocated string and it must be freed by xmpp_free().

Without this patch I can't join to a conference. Every time profanity crashes at xmpp/stanza.c:594 with the following input message: http://pastebin.com/egPpZr1Q (exactly on 'media' tag).