Use more explicit way of copying pointer value to a buffer

Use more explicit way of copying pointer value to a buffer

The code initializing GMK Counter uses the group pointer value as extra
entropy and to distinguish different group instances. Some static
analyzers complain about the sizeof(pointer) with memcpy, so use a more
explicit type casting to make it more obvious what the code is doing.

Signed-hostap: Jouni Malinen <j@w1.fi>