http2: Fix assertion error on redirect with CL=0

This fixes assertion error which occurs when redirect is done with 0
length body via HTTP/2, and the easy handle is reused, but new
connection is established due to hostname change:

    curl: http2.c:1572: ssize_t http2_recv(struct connectdata *,
      int, char *, size_t, CURLcode *):
    Assertion `httpc->drain_total >= data->state.drain' failed.

To fix this bug, ensure that http2_handle_stream is called.

Fixes #1286
Closes #1302