Fix for the boundary case in which each character of an SSI directive

Fix for the boundary case in which each character of an SSI directive
is in a separate bucket...the code in send_parsed_content() doesn't
expect a case where ctx->state==PARSE_DIRECTIVE but ctx->tag_start_buffer
is NULL.  So in found_start_sequence(), we set ctx->tag_start_buffer
to point to the next bucket if the end of the <!--# is at the very
end of the current bucket.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@94454 13f79535-47bb-0310-9956-ffa450edef68