Fix random range for XFunctionDescriptions::testGetById

...regression introduced with eff70347190a6642fd62a9e0b20e4366c39fbc7a
"tdf#45904 Move _XFunctionDescriptions Java test to C++" (where
java.util.Random.nextInt(n) returned a value 0 <= x < n), apparently causing
sporadic failures of CppunitTest_sc_functionlistobj like <https://
ci.libreoffice.org/job/lo_gerrit/27696/Config=linux_clang_dbgutil_64/console>

> ##Failure Location unknown## : Error
> Test name: sc_apitest::ScFunctionListObj::testGetById
> An uncaught exception of type com.sun.star.lang.IndexOutOfBoundsException

Also verify that nCount is not negative, so that the requirements of the
std::uniform_int_distribution ctor are guaranteed to be satisfied.

Change-Id: I90a5dc234fdd07f52fea69ae1d406f0818efb007