Fix aborts triggered by _FORTIFY_SOURCE

[bk138]

Both Android and Flatpak build systems by default use _FORTIFY_SOURCE=2 which causes aborts when using invalid sockets in FD_SET, FD_CLR and FD_ISSET, see https://www.gnu.org/software/libc/manual/html_node/Source-Fortification.html

• check for invalid sockets before FD_SET, FD_CLR and FD_ISSET
• protect check and socket usage w/ mutex