HTTP/2: discard remaining request body after redirect.

Previously, if unbuffered request body reading wasn't finished before
the request was redirected to a different location using error_page
or X-Accel-Redirect, and the request body is read again, this could
lead to disastrous effects, such as a duplicate post_handler call or
"http request count is zero" alert followed by a segmentation fault.

This happened in the following configuration (ticket #1819):

    location / {
        proxy_request_buffering off;
        proxy_pass http://bad;
        proxy_intercept_errors on;
        error_page 502 = /error;
    }

    location /error {
        proxy_pass http://backend;
    }

src/http/v2/ngx_http_v2.c

@@ -947,15 +947,22 @@ ngx_http_v2_state_read_data(ngx_http_v2_connection_t *h2c, u_char *pos,
         return ngx_http_v2_state_skip_padded(h2c, pos, end);
     }
 
+    r = stream->request;
+
+    if (r->reading_body && !r->request_body_no_buffering) {
+        ngx_log_debug0(NGX_LOG_DEBUG_HTTP, h2c->connection->log, 0,
+                       "skipping http2 DATA frame");
+
+        return ngx_http_v2_state_skip_padded(h2c, pos, end);
+    }
+
     size = end - pos;
 
     if (size >= h2c->state.length) {
         size = h2c->state.length;
         stream->in_closed = h2c->state.flags & NGX_HTTP_V2_END_STREAM_FLAG;
     }
 
-    r = stream->request;
-
     if (r->request_body) {
         rc = ngx_http_v2_process_request_body(r, pos, size, stream->in_closed);