qemu_nbdkit: fix possible null dereference

Function virGetConnectSecret() can return NULL so we need to check it
since in virSecretGetSecretString() it gets dereferenced.

Reported-by: coverity
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

src/qemu/qemu_nbdkit.c

@@ -971,6 +971,9 @@ qemuNbdkitProcessBuildCommandAuth(virStorageAuthDef *authdef,
     }
 
     conn = virGetConnectSecret();
+    if (!conn)
+        return -1;
+
     if (virSecretGetSecretString(conn,
                                  &authdef->seclookupdef,
                                  secrettype,