[rdf] fix windows warning about precision lost in RDisplay::fWidth

Here size_t variable converted into unsigned short - windows complains.
Original message:

RDFDisplay.cxx(96): warning C4267: '=': conversion from 'size_t' to
'_Ty', possible loss of data 

RDFDisplay.cxx(117): warning C4267: '=': conversion from 'size_t' to
'_Ty', possible loss of data

tree/dataframe/inc/ROOT/RDF/RDisplay.hxx

@@ -183,6 +183,8 @@ private:
    /// If the number of required rows has been parsed, returns false.
    bool HasNext() { return fEntries > 0; }
 
+   void EnsureCurrentColumnWidth(size_t w);
+
 public:
    ////////////////////////////////////////////////////////////////////////////
    /// Creates an RDisplay to print the event values

tree/dataframe/src/RDFDisplay.cxx

@@ -2,6 +2,7 @@
 #include "TInterpreter.h"
 
 #include <iomanip>
+#include <limits>
 
 namespace ROOT {
 namespace Internal {
@@ -89,12 +90,23 @@ bool RDisplayElement::IsEmpty() const
 } // namespace Internal
 
 namespace RDF {
-void RDisplay::AddToRow(const std::string &stringEle)
+
+void RDisplay::EnsureCurrentColumnWidth(size_t w)
 {
    // If the current element is wider than the widest element found, update the width
-   if (fWidths[fCurrentColumn] < stringEle.length()) {
-      fWidths[fCurrentColumn] = stringEle.length();
+   if (fWidths[fCurrentColumn] < w) {
+      if (w > std::numeric_limits<unsigned short>::max()) {
+         w = std::numeric_limits<unsigned short>::max();
+      }
+      fWidths[fCurrentColumn] = (unsigned short) w;
    }
+}
+
+void RDisplay::AddToRow(const std::string &stringEle)
+{
+   // If the current element is wider than the widest element found, update the width
+   EnsureCurrentColumnWidth(stringEle.length());
+
    // Save the element...
    fTable[fCurrentRow][fCurrentColumn] = DElement_t(stringEle);
 
@@ -113,18 +125,14 @@ void RDisplay::AddCollectionToRow(const std::vector<std::string> &collection)
       auto element = DElement_t(stringEle);
 
       // Update the width if this element is the biggest found
-      if (fWidths[fCurrentColumn] < stringEle.length()) {
-         fWidths[fCurrentColumn] = stringEle.length();
-      }
+      EnsureCurrentColumnWidth(stringEle.length());
 
       if (index == 0 || index == collectionSize - 1) {
          // Do nothing, by default DisplayElement is printed
       } else if (index == 1) {
          element.SetDots();
          // Be sure the "..." fit
-         if (fWidths[fCurrentColumn] < 3) {
-            fWidths[fCurrentColumn] = 3;
-         }
+         EnsureCurrentColumnWidth(3);
       } else {
          // In the Print(), after the dots, all element will just be ignored except the last one.
          element.SetIgnore();