fix(ubsan): MCOL-5844 - iron out UBSAN reports

The most important fix here is the fix of possible buffer overrun in
DATEFORMAT() function. A "%W" format, repeated enough times, would
overflow the 256-bytes buffer for result. Now we use ostringstream to
construct result and we are safe.

Changes in date/time projection functions made me fix difference between
us and server behavior. The new, better behavior is reflected in changes
in tests' results.

Also, there was incorrect logic in TRUNCATE() and ROUND() functions in
computing the decimal "shift."

dbcon/execplan/arithmeticoperator.h

@@ -323,6 +323,19 @@ inline void ArithmeticOperator::evaluate(rowgroup::Row& row, bool& isNull, Parse
 template <typename result_t>
 inline result_t ArithmeticOperator::execute(result_t op1, result_t op2, bool& isNull)
 {
+  if (isNull)
+  {
+    // at least one operand is NULL.
+    // do nothing, return 0.
+    if constexpr (std::is_same<T, datatypes::TSInt128>::value)
+    {
+      return datatypes::TSInt128();  // returns 0
+    }
+    else
+    {
+      return T{0};
+    }
+  }
   switch (fOp)
   {
     case OP_ADD: return op1 + op2;