localauth plugin: fix coverity warning
    
    Error: FORWARD_NULL (CWE-476): [#def1]
    sssd-1.12.90/src/krb5_plugin/sssd_krb5_localauth_plugin.c:111: assign_zero: Assigning: "pwd.pw_name" = "NULL".
    sssd-1.12.90/src/krb5_plugin/sssd_krb5_localauth_plugin.c:142: var_deref_model: Passing null pointer "pwd.pw_name" to "strdup", which dereferences it.
      140|       }
      141|
      142|->     str = strdup(pwd.pw_name);
      143|       if (str == NULL) {
      144|           ret = ENOMEM;
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>