FastCGI: adjust buffer position when parsing incomplete records.

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.