omit unnecessary conditions

when the FastCGI header was split in records,
nginx passed garbage in the header to a client

if subrequest response was buffered in file, then subrequest was finalized
with 0 code, ngx_http_writer() was not set, and response part in file was lost

if a subrequest was finished and SSI was in middle of SSI command parsing
and the command was a fast subrequest then the second subrequest output
was just after first subrequest output and response part of main request
between the subrequests went after the second subrequest.

if "<!--" was on edge of buf, then ssi_sharp_state was saved in ctx->saved
and if it was not reset, then any '#' was treated as SSI command start