BUG/MINOR: server: fix memory leak on "default-server" parsing failures

BUG/MINOR: server: fix memory leak on "default-server" parsing failures

This is a fix for very minor bug which may be triggerd only at parsing time
upon parsing failures (allocation failure, or default server name mismatch).

When parsing a named "default-server" instance in _srv_parse_init(), <name> is
allocated via strdup(). If a name mismatch occurs or if srv_alloc() fails,
the function jumps to the <out> label without freeing this string.

Fix this by initializing <name> to NULL at the function start and calling
free(name) in the <out> cleanup block.

This issue was reported by GH #3462.

No need to backport.