Avoid unnecessary memset to zero, snprintf always puts a
trailing '\0'.  We also have no need to subtract one from the
buffer we're snprintf-ing it to.

We also check the return value of snprintf to ensure it's not
too long.  I have a feeling we might as well avoid snprintf
altogether so we don't have to worry about buffer sizing/stack
overflow and just do a bunch of write(2)s, letting Nagle sort it
out...

Also, centralize some of the exit error handling in with
goto.  This makes the code a bit more consistent and
maintainable as well as reducing code and binary size.

git-svn-id: https://svn.musicpd.org/mpd/trunk@5395 09075e82-0dd4-0310-85a5-a0d7c8717e4f