Something is wrong in 'userp' for the HTTP2 recv_callback(). The
session is created using bogus user-data; '&conn' and not 'conn'.
I noticed this since the socket-value in Curl_read_plain() was set to a
impossible high value.
Using the first little merge of nghttp2 into libcurl, I stumbeled on the
missing 'snprintf' in MSVCRT. Isn't this how we do it for other libcurl
files? I.e. use 'curl_msnprintf' and not 'snprintf' directly: