input/ar: remove unused variable
stream: silence clang empty statement warnings

clang printed warnings like:

stream/stream.c:692:65: warning: if statement has empty body [-Wempty-body]
            GET_UTF16(c, src < end - 1 ? get_le16_inc(&src) : 0,;

This macro expands to "if(cond) ;". Replace it with an empty statement
that doesn't lead to a clang warning.
demux_mf: fix crashes when '*' matches directories

Doing 'mpv mf://*' in a file with directories would crash, because even
though directories are skipped, the corresponding file entry is just
left at NULL, leading to a segfault on access. So explicitly skip NULL
entries.