Fix /omemo fingerprint crashing in a MUC

Reported by a user in the profanity MUC.

Fixup of e1d137f4e6 resp. #1863

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

src/command/cmd_funcs.c

@@ -8887,7 +8887,7 @@ cmd_omemo_fingerprint(ProfWin* window, const char* const command, gchar** args)
         return TRUE;
     }
 
-    auto_jid Jid* jid;
+    auto_jid Jid* jid = NULL;
     if (!args[1]) {
         if (window->type == WIN_CONSOLE) {
             auto_char char* fingerprint = omemo_own_fingerprint(TRUE);