tree: fix segfault in nvme_free_tree()

Commands like nvme list & list-subsys currently segfault for --help or
any invalid command option as shown below:

nvme list -h
Usage: nvme list <device> [OPTIONS]

Retrieve basic information for all NVMe namespaces

Options:
  [  --verbose, -v ]                    --- output verbosity
  [  --output-format=<FMT>, -o <FMT> ]  --- format: normal|json|binary
Segmentation fault (core dumped)

This is due to an invalid dereferencing of the nvme_root_t object in
nvme_free_tree(). Fix this by checking whether this object is valid
before dereferencing and freeing it. And while we are it, ensure
r->options is also valid before freeing it.

Signed-off-by: Martin George <marting@netapp.com>

src/nvme/tree.c

@@ -377,7 +377,11 @@ void nvme_free_tree(nvme_root_t r)
 {
 	struct nvme_host *h, *_h;
 
-	free(r->options);
+	if (!r)
+		return;
+
+	if (r->options)
+		free(r->options);
 	nvme_for_each_host_safe(r, h, _h)
 		__nvme_free_host(h);
 	if (r->config_file)