udp.c: Add explicit cast to muffle bogus Clang warning.

udp.c: Add explicit cast to muffle bogus Clang warning.

Clang is complaining (`-Wsign-compare') about the comparison between
`salen' (`socklen_t', i.e., an `int' with a false moustache) and
`size_t' (`unsigned int' in this case).  I can see that some warnings of
this kind are useful, but not this one.  The usual arithmetic
conversions apply, so `salen' is converted to `size_t'.  If it was
negative before, it's now very positive, which will trip the the
comparison and call `FAIL' -- which seems like a plausible outcome.

Muffle the warning by adding an explicit cast.  This is ugly and
pointless, though: other suggestions are welcome.

Signed-off-by: Mark Wooding <mdw@distorted.org.uk>
Acked-by: Ian Jackson <ijackson@chiark.greenend.org.uk>