coverity#1352448 Use of untrusted scalar value

and

coverity#1352449 Use of untrusted scalar value

I suspect this will fix the warning, given that "Casting narrower unsigned
pBuffer[i] to wider signed type short effectively tests its lower bound", so
if we pass it though as-is we probably quieten these

Change-Id: I2ece08f7301f5d548d905d221e5a6af0da51a2bb

sal/qa/rtl/random/rtl_random.cxx

@@ -198,9 +198,8 @@ public:
     }
     ~Statistics(){}
 
-    void addValue(sal_Int16 _nIndex, sal_Int32 _nValue)
+    void addValue(sal_uInt8 _nIndex, sal_Int32 _nValue)
     {
-        OSL_ASSERT(_nIndex >= 0 && _nIndex < 256);
         m_nDispensation[_nIndex] += _nValue;
     }