MINOR: hlua: fix ambiguous hlua usage in hlua_filter_delete()

In GH #2804, @Bbulatov reported that the result of hlua_stream_ctx_get()
was used and de-referenced without checking if it's NULL in
hlua_filter_delete() while other functions used to check for NULL before
de-referencing it.

In fact hlua_stream_ctx_get() can only return NULL if
hlua_stream_ctx_prepare() failed or was not called on the current stream.

Now because of the filter's API, since hlua_filter_delete() is mapped as
detach method and hlua_filter_new() as attach method, and since
hlua_filter_new() is responsible for calling hlua_stream_ctx_prepare(),
there's no reason hlua_filter_delete() should be called if
hlua_filter_new() failed or wasn't called. Thus we can assume that hlua
can never be NULL in hlua_filter_delete(), so we add a BUG_ON() to ensure
it is always the case and remove the ambiguity.

src/hlua.c

@@ -12310,6 +12310,10 @@ static void hlua_filter_delete(struct stream *s, struct filter *filter)
 	struct hlua_flt_ctx *flt_ctx = filter->ctx;
 	struct hlua *hlua = hlua_stream_ctx_get(s, flt_ctx->_hlua->state_id);
 
+	BUG_ON(!hlua); /* hlua_filter_new() is responsible for preparing
+	                * hlua stream_ctx, thus hlua_stream_ctx_get() should
+	                * not return NULL!
+	                */
 	hlua_lock(hlua);
 	hlua_unref(hlua->T, flt_ctx->ref);
 	hlua_unlock(hlua);