FastCGI: fixed zero size buf alerts on extra data (ticket #2018).

After 05e42236e95b (1.19.1) responses with extra data might result in
zero size buffers being generated and "zero size buf" alerts in writer
(if f->rest happened to be 0 when processing additional stdout data).

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 e50d1a7..5191880 100644
--- a/src/http/modules/ngx_http_fastcgi_module.c
+++ b/src/http/modules/ngx_http_fastcgi_module.c
@@ -2306,6 +2306,18 @@
             break;
         }
 
+        if (f->rest == -2) {
+            f->rest = r->upstream->headers_in.content_length_n;
+        }
+
+        if (f->rest == 0) {
+            ngx_log_error(NGX_LOG_WARN, p->log, 0,
+                          "upstream sent more data than specified in "
+                          "\"Content-Length\" header");
+            p->upstream_done = 1;
+            break;
+        }
+
         cl = ngx_chain_get_free_buf(p->pool, &p->free);
         if (cl == NULL) {
             return NGX_ERROR;
@@ -2349,11 +2361,7 @@
             b->last = f->last;
         }
 
-        if (f->rest == -2) {
-            f->rest = r->upstream->headers_in.content_length_n;
-        }
-
-        if (f->rest >= 0) {
+        if (f->rest > 0) {
 
             if (b->last - b->pos > f->rest) {
                 ngx_log_error(NGX_LOG_WARN, p->log, 0,
@@ -2564,6 +2572,14 @@
             break;
         }
 
+        if (f->rest == 0) {
+            ngx_log_error(NGX_LOG_WARN, r->connection->log, 0,
+                          "upstream sent more data than specified in "
+                          "\"Content-Length\" header");
+            u->length = 0;
+            break;
+        }
+
         cl = ngx_chain_get_free_buf(r->pool, &u->free_bufs);
         if (cl == NULL) {
             return NGX_ERROR;
@@ -2594,7 +2610,7 @@
             b->last = f->last;
         }
 
-        if (f->rest >= 0) {
+        if (f->rest > 0) {
 
             if (b->last - b->pos > f->rest) {
                 ngx_log_error(NGX_LOG_WARN, r->connection->log, 0,