conn: don't crash when user sets password to NULL

Make it possible to reset password to NULL. It is not required for
ANONYMOUS authentication. Also, report an error and disconnect if
password is not set and libstrophe should try authentication mechanisms
other than ANONYMOUS.

src/auth.c

@@ -667,6 +667,10 @@ static void _auth(xmpp_conn_t *const conn)
         xmpp_error(conn->ctx, "auth",
                    "No node in JID, and SASL ANONYMOUS unsupported.");
         xmpp_disconnect(conn);
+    } else if (conn->pass == NULL) {
+        xmpp_error(conn->ctx, "auth",
+                   "Password hasn't been set, and SASL ANONYMOUS unsupported.");
+        xmpp_disconnect(conn);
     } else if (conn->sasl_support & SASL_MASK_SCRAM) {
         scram_ctx = xmpp_alloc(conn->ctx, sizeof(*scram_ctx));
         if (conn->sasl_support & SASL_MASK_SCRAMSHA512)

src/conn.c

@@ -420,7 +420,7 @@ void xmpp_conn_set_pass(xmpp_conn_t *const conn, const char *const pass)
 {
     if (conn->pass)
         xmpp_free(conn->ctx, conn->pass);
-    conn->pass = xmpp_strdup(conn->ctx, pass);
+    conn->pass = pass ? xmpp_strdup(conn->ctx, pass) : NULL;
 }
 
 /** Get the strophe context that the connection is associated with.