BUG/MEDIUM: mux-h2: erase h2c->wait_event.tasklet on error path

On the allocation error path in h2_init() we may check if
h2c->wait_event.tasklet needs to be released but it has not yet been
zeroed. Let's do this before jumping to the freeing location.

This needs to be backported to all maintained versions.
diff --git a/src/mux_h2.c b/src/mux_h2.c
index 1d4dd3a..8697799 100644
--- a/src/mux_h2.c
+++ b/src/mux_h2.c
@@ -962,6 +962,7 @@
 
 	h2c->proxy = prx;
 	h2c->task = NULL;
+	h2c->wait_event.tasklet = NULL;
 	h2c->idle_start = now_ms;
 	if (tick_isset(h2c->timeout)) {
 		t = task_new_here();