mutliplex protocol thread safety: stop_expose_to and parameter push_value segfaults

[x37v]

Here is an example test that causes the crash:

https://github.com/Cycling74/libossia/blob/xnor/multiplex-test/tests/Network/MultiplexTest.cpp#L143-L227

When I run some code in a debugger I see a segfault when I push_value and the multiplex protocol iterates its protocols.

[x37v]

Hey @jcelerier I think you said you were going to add some notes here to maybe point someone (me) in the right direction as far as resolving this issue and contributing back to the project?

[x37v]

BTW.. the c docs claim that ossia_protocol_multiplex_expose_to is MT safe.. it pushes to a vector which is also accessed from other threads.. the docs from std::vector AFAIK don't indicate that push_back is thread safe..