MINOR: applet: Add function to release appctx on error during init stage

MINOR: applet: Add function to release appctx on error during init stage

appctx_free_on_early_error() must be used to release a freshly created
frontend appctx if an error occurred during the init stage. It takes care to
release the stream instead of the appctx if it exists. For a backend appctx,
it just calls appctx_free().