"/bookmark add foo" crash - fix

A user providing an invalid JID when creating a new bookmark (like 'foo')
would reproducibly crash/segfault profanity, as it insists on checking
string length behind the @ of the JID. However, it could be NULL
if the user accidentally omitted it.
The patch avoids the crash by NULL-checking and prevents
getting there in the first place by checking the argument to "add".
Backtrace of unpatched profanity with above command:

Program received signal EXC_BAD_ACCESS, Could not access memory.
Reason: KERN_INVALID_ADDRESS at address: 0x0000000000000000
0x00007fff85699732 in strlen ()
(gdb) bt
 #0  0x00007fff85699732 in strlen ()
 #1  0x00000001000965d2 in xmpp_strdup ()
 #2  0x0000000100095d6d in xmpp_stanza_set_attribute ()
 #3  0x0000000100011c5c in _send_bookmarks ()
 #4  0x00000001000115a8 in _bookmark_add ()
 #5  0x000000010003320d in cmd_bookmark ()
 #6  0x000000010002a0f2 in cmd_execute ()
 #7  0x0000000100003a1d in process_input ()
 #8  0x00000001000037c7 in prof_run ()
 #9  0x0000000100045032 in main ()
(gdb)