BUG/MEDIUM: proxy: ignore stopped peers

BUG/MEDIUM: proxy: ignore stopped peers

Since commit f83d3fe ("MEDIUM: init: stop any peers section not bound
to the correct process"), it is possible to stop unused peers on certain
processes. The problem is that the pause/resume/stop functions are not
aware of this and will pass a NULL proxy pointer to the respective
functions, resulting in segfaults in unbound processes during soft
restarts.

Properly check that the peers' frontend is still valid before calling
them.

This bug also affects 1.5 so the fix must be backported. Note that this
fix is not enough to completely get rid of the segfault, the next one
is needed as well.