BUG/MEDIUM: muxes: Don't dereference mux context if null in release functions

When a mux context is released, we must be sure it exists before dereferencing
it. The bug was introduced in the commit 39a96ee16 ("MEDIUM: muxes: Be prepared
to don't own connection during the release").

No need to backport this patch, expect if the commit 39a96ee16 is backported
too.
3 files changed