Fix: don't store an int64_t value in a ptr

Fix: don't store an int64_t value in a ptr

The key of the stream_classes GHashTable was a int64_t stored directly
in a pointer which works only on 64bits platform. Instead, allocate a
int64_t value and use a pointer to it as the key.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>