Fixed potential segfault in ngx_http_keepalive_handler().

In case of error in the read event handling we close a connection
by calling ngx_http_close_connection(), that also destroys connection
pool. Thereafter, an attempt to free a buffer (added in r4892) that
was allocated from the pool could cause SIGSEGV and is meaningless
as well (the buffer already freed with the pool).


git-svn-id: svn://svn.nginx.org/nginx/trunk@5083 73f98a42-aea0-e011-b76d-00259023448c