http2: Use size_t type for data drain count

Ref: https://github.com/curl/curl/issues/659
Ref: https://github.com/curl/curl/pull/663

lib/http.h

@@ -214,7 +214,7 @@ struct http_conn {
      them for both cases. */
   int32_t pause_stream_id; /* stream ID which paused
                               nghttp2_session_mem_recv */
-  int drain_total; /* sum of all stream's UrlState.drain */
+  size_t drain_total; /* sum of all stream's UrlState.drain */
 
   /* this is a hash of all individual streams (SessionHandle structs) */
   struct h2settings settings;

lib/http2.c

@@ -1166,6 +1166,7 @@ static ssize_t http2_handle_stream_close(struct connectdata *conn,
     httpc->pause_stream_id = 0;
   }
 
+  DEBUGASSERT(httpc->drain_total >= data->state.drain);
   httpc->drain_total -= data->state.drain;
   data->state.drain = 0;
 
@@ -1471,6 +1472,7 @@ static ssize_t http2_recv(struct connectdata *conn, int sockindex,
                    stream->stream_id));
     }
     else if(!stream->closed) {
+      DEBUGASSERT(httpc->drain_total >= data->state.drain);
       httpc->drain_total -= data->state.drain;
       data->state.drain = 0; /* this stream is hereby drained */
     }

lib/urldata.h

@@ -1314,9 +1314,9 @@ struct UrlState {
   curl_off_t infilesize; /* size of file to upload, -1 means unknown.
                             Copied from set.filesize at start of operation */
 
-  int drain; /* Increased when this stream has data to read, even if its
+  size_t drain; /* Increased when this stream has data to read, even if its
-                socket not necessarily is readable. Decreased when
+                   socket is not necessarily is readable. Decreased when
-                checked. */
+                   checked. */
   bool done; /* set to FALSE when Curl_do() is called and set to TRUE when
                 Curl_done() is called, to prevent Curl_done() to get invoked
                 twice when the multi interface is used. */