Fix: uprobe: inequality comparison against NULL

The expression will not have its intended effect.

In lttng_userspace_probe_location_function_serialize: Comparing a
pointer against NULL using an operator such as < or >=.

`binary_fd` is now a fd_handle instance rather than a "raw" fd. All
instances of `binary_fd` are renamed to `binary_fd_handle` to prevent
such errors in the future.

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I57f1a3d5a01029084889a183881fac6f1fe9b6d9