FastCGI: skip special bufs in buffered request body chain.

This prevents forming empty records out of such buffers.  Particularly it fixes
double end-of-stream records with chunked transfer encoding, or when HTTP/2 is
used and the END_STREAM flag has been sent without data.  In both cases there
is an empty buffer at the end of the request body chain with the "last_buf"
flag set.

The canonical libfcgi, as well as php implementation, tolerates such records,
while the HHVM parser is more strict and drops the connection (ticket #950).

src/http/modules/ngx_http_fastcgi_module.c

diff --git a/src/http/modules/ngx_http_fastcgi_module.c b/src/http/modules/ngx_http_fastcgi_module.c
index a861203..2d288ce 100644
--- a/src/http/modules/ngx_http_fastcgi_module.c
+++ b/src/http/modules/ngx_http_fastcgi_module.c
@@ -1177,6 +1177,11 @@
 
         while (body) {
 
+            if (ngx_buf_special(body->buf)) {
+                body = body->next;
+                continue;
+            }
+
             if (body->buf->in_file) {
                 file_pos = body->buf->file_pos;