after a call to nghttp2_session_mem_send_internal() which should
set it, however in nghttp2_session_mem_send_internal() it is
possible to return before setting the pointer.

This change initializes the variable to NULL where delcared and
sets the variable in nghttp2_session_mem_send_internal() to
NULL before possibly returning rather than after.

both options are not necessary but are both ideal practice