GUACAMOLE-400: Fix guacd crash when ssh key fails

Root Cause:
In the ssh library of guacd, function ssh_client_thread(), when guac_ssh_get_user() fails to load private key for ssh authentication, it will return NULL. In this case, the subsequent call to guac_common_ssh_create_session() with parameter 'user=0x0' will cause guacd crash in function guac_common_ssh_authenticate() by accessing 'user->username'.

Solution:
- Update the comment of function guac_ssh_get_user() to document that NULL will be returned if fails to import key for the user.
- In function ssh_client_thread(), verify the return of guac_ssh_get_user(). If ssh_client->user is NULL, return NULL.

Test:
- Configured a ssh app with an encrypted private key and a wrong passphrase.
- Ran the ssh app from web portal and observed guacd crash.
- Applied the fix and reran the ssh app. Observed no crash.

src/protocols/ssh/ssh.c

@@ -63,7 +63,8 @@
  *     terminal to use when prompting the user.
  *
  * @return
- *     A new user object containing the user's username and other credentials.
+ *     A new user object containing the user's username and other credentials,
+ *     or NULL if fails to import key.
  */
 static guac_common_ssh_user* guac_ssh_get_user(guac_client* client) {
 
@@ -215,6 +216,10 @@ void* ssh_client_thread(void* data) {
 
     /* Get user and credentials */
     ssh_client->user = guac_ssh_get_user(client);
+    if (ssh_client->user == NULL) {
+        /* Already aborted within guac_ssh_get_user() */
+        return NULL;
+    }
 
     /* Open SSH session */
     ssh_client->session = guac_common_ssh_create_session(client,