Fix a couple of uninitialised variables.

throttled_by_backlog is quite new, so it's not too surprising that I
hadn't already noticed it wasn't initialised. But the failure to null
out the final 'next' pointer in the callbacks list when it's rewritten
as a result of delete_callbacks_for_context is a great deal older, so
I'm a lot more puzzled about how it hasn't come up until now!

callback.c

@@ -65,6 +65,7 @@ void delete_callbacks_for_context(void *ctx)
 
     cbhead = newhead;
     cbtail = newtail;
+    newtail->next = NULL;
 }
 
 void queue_toplevel_callback(toplevel_callback_fn_t fn, void *ctx)

ssh2connection.c

@@ -1229,6 +1229,7 @@ void ssh2_channel_init(struct ssh2_channel *c)
     c->closes = 0;
     c->pending_eof = FALSE;
     c->throttling_conn = FALSE;
+    c->throttled_by_backlog = FALSE;
     c->sharectx = NULL;
     c->locwindow = c->locmaxwin = c->remlocwin =
         s->ssh_is_simple ? OUR_V2_BIGWIN : OUR_V2_WINSIZE;