http2: Fix address sanitizer memcpy warning

- In Curl_http2_switched don't call memcpy when src is NULL.

Curl_http2_switched can be called like:

Curl_http2_switched(conn, NULL, 0);

.. and prior to this change memcpy was then called like:

memcpy(dest, NULL, 0)

.. causing address sanitizer to warn:

http2.c:2057:3: runtime error: null pointer passed as argument 2, which
is declared to never be null