Added size check to ngx_http_alloc_large_header_buffer().

Added size check to ngx_http_alloc_large_header_buffer().

This ensures that copying won't write more than the buffer size
even if the buffer comes from hc->free and it is smaller than the large
client header buffer size in the virtual host configuration.  This might
happen if size of large client header buffers is different in name-based
virtual hosts, similarly to the problem with number of buffers fixed
in 6926:e662cbf1b932.