Fix random range for XFunctionDescriptions::testGetById

...regression introduced with eff70347
"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

test/source/sheet/xfunctiondescriptions.cxx

@@ -30,12 +30,12 @@ void XFunctionDescriptions::testGetById()
     uno::Reference<sheet::XFunctionDescriptions> xFD(init(), UNO_QUERY_THROW);
 
     const sal_Int32 nCount = xFD->getCount();
-    CPPUNIT_ASSERT_MESSAGE("No FunctionDescriptions available", 0 != nCount);
+    CPPUNIT_ASSERT_MESSAGE("No FunctionDescriptions available", 0 < nCount);
 
     // first grab a random function descriptions
     std::random_device rd;
     std::mt19937 gen(rd());
-    std::uniform_int_distribution<> distr(1, nCount);
+    std::uniform_int_distribution<> distr(0, nCount - 1);
     int nNumber = distr(gen);
 
     sal_Int32 aId1 = 0;