ServerSocket: Initialize length variable
decoder/Opus: replace non-static data member initializers

Would require gcc 4.7, and MPD attempts to be compatible with gcc 4.6.
event/ServerSocket: close only sockets that have been opened

Fixes assertion failure.
output/alsa: default "device" is an empty string

Fixes nullptr dereference in std::string::operator=().  Also fixes a
memory leak by using config_get_block_string() instead of
config_dup_block_string().
SocketUtil: add separators to g_prefix_error() call
output/alsa: move code to alsa_write_silence()
output/alsa: pre-allocate the silence buffer
output/alsa: workaround for noise after manual song change

Workaround for driver bug observed on the Raspberry Pi, see code
comment for details.