FastCGI: adjust buffer position when parsing incomplete records. Previously, nginx failed to move buffer position when parsing an incomplete record header, and due to this wasn't be able to continue parsing once remaining bytes of the record header were received. This can affect response header parsing, potentially generating spurious errors like "upstream sent unexpected FastCGI request id high byte: 1 while reading response header from upstream". While this is very unlikely, since usually record headers are written in a single buffer, this still can happen in real life, for example, if a record header will be split across two TCP packets and the second packet will be delayed. This does not affect non-buffered response body proxying, due to "buf->pos = buf->last;" at the start of the ngx_http_fastcgi_non_buffered_filter() function. Also this does not affect buffered response body proxying, as each input buffer is only passed to the filter once.