Fix dangling else warning in Server::msgACL()
Fix dangling else warning in Server::msgACL()

This commit fixes a warning encountered today on FreeBSD, probably because Clang was updated and now `-Wdangling-else` is enabled by default.

Messages.cpp:1330:5: error: add explicit braces to avoid dangling else [-Werror,-Wdangling-else]
                                foreach(int id, pg->members()) {
                                ^

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