pow5mult() is not reentrant therefore QString::toDouble() is not either

From this backtrace it is apparent that QString:toDouble() can eventually call pow5mult(). (line numbers are from 4.8.5 but it looks like 5.2.1 has the same issue).

==649== at 0xB27F5BD: pow5mult(Bigint*, int) (qlocale_tools.cpp:1122)
==649== by 0xB280DA6: qstrtod(char const*, char const*, bool) (qlocale_tools.cpp:1908)
==649== by 0xB27A236: QLocalePrivate::bytearrayToDouble(char const*, bool*, bool*) (qlocale.cpp:3032)
==649== by 0xB279DC4: QLocalePrivate::stringToDouble(QString const&, bool*, QLocalePrivate::GroupSeparatorMode) const (qlocale.cpp:2976)
==649== by 0xB2AD306: QString::toDouble(bool*) const (qstring.cpp:5870)

pow5mult() uses a static variable called p5s with no protection so pow5mult is not reentrant therefore QString::toDouble() is not reentrant as claimed in the docs.