Fix Clang warning of constant-false comparison

qstringvaluefns.cpp:224:23: error: comparison of constant -1 with expression of type 'QString::NormalizationForm' is always false [-Werror,-Wtautological-constant-out-of-range-compare]

Change-Id: Ia0dd98f72521709712f5ed11ecf19a48acaf1998
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>