[XERCESC-2241] Fix integer overflows in DFAContentModel class

On .xsd files like the following ones (generated by ossfuzz, so broken),
integer overflows can happen in DFAContentModel::countLeafNodes() and
DFAContentModel::buildDFA() which can later cause out-of-bounds access.

Found in https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=52025

```
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
           xmlns:myns="http://myns"
           targetNamespace="http://myns"
           elementFormDefault="qualified" attributeFormDefault="unqualified">

<xs:element name="main_elt">
  <xs:complexType>
     <xs:sequence>
        <xs:group ref="myns:mygroup" minOccurs="32767" maxOccurs="1"/>
      </xs:sequence>
  </xs:complexType>
</xs:element>

<xs:group name="mygroup">
  <xs:sequence>
      <!-- related to https://issues.apache.org/jira/browse/XERCESC-1051 -->
      <xs:element name="elt" maxOccurs="33333">
        <xs:complexType>
            <xs:sequence>
 ame="x" type="xs:int" maxOccurs="1"/>
            </xs:sequence>
        </xs:complexType>
      </xs:element>
  </xs:sequence>
</xs:group>

</xs:schema>
```

src/xercesc/validators/common/DFAContentModel.cpp

@@ -42,6 +42,7 @@
 #include <xercesc/util/RefHashTableOf.hpp>
 #include <xercesc/util/XMLInteger.hpp>
 #include <math.h>
+#include <limits>
 
 namespace XERCES_CPP_NAMESPACE {
 
@@ -661,8 +662,15 @@ void DFAContentModel::buildDFA(ContentSpecNode* const curNode)
     //  in the fLeafCount member.
     //
     fLeafCount=countLeafNodes(curNode);
+    // Avoid integer overflow in below fLeafCount++ increment
+    if (fLeafCount > std::numeric_limits<unsigned int>::max() - 1)
+        throw OutOfMemoryException();
     fEOCPos = fLeafCount++;
 
+    // Avoid integer overflow in below memory allocatoin
+    if (fLeafCount > std::numeric_limits<size_t>::max() / sizeof(CMLeaf*))
+        throw OutOfMemoryException();
+
     //  We need to build an array of references to the non-epsilon
     //  leaf nodes. We will put them in the array according to their position values
     //
@@ -1364,14 +1372,27 @@ unsigned int DFAContentModel::countLeafNodes(ContentSpecNode* const curNode)
         if(nLoopCount!=0)
         {
             count += countLeafNodes(cursor);
-            for(unsigned int i=0;i<nLoopCount;i++)
-                count += countLeafNodes(rightNode);
+            const unsigned int countRight = countLeafNodes(rightNode);
+            // Avoid integer overflow in below multiplication
+            if (countRight > std::numeric_limits<unsigned int>::max() / nLoopCount)
+                throw OutOfMemoryException();
+            const unsigned int countRightMulLoopCount = nLoopCount * countRight;
+            // Avoid integer overflow in below addition
+            if (count > std::numeric_limits<unsigned int>::max() - countRightMulLoopCount)
+                throw OutOfMemoryException();
+            count += countRightMulLoopCount;
             return count;
         }
         if(leftNode)
             count+=countLeafNodes(leftNode);
         if(rightNode)
-            count+=countLeafNodes(rightNode);
+        {
+            const unsigned int countRight = countLeafNodes(rightNode);
+            // Avoid integer overflow in below addition
+            if (count > std::numeric_limits<unsigned int>::max() - countRight)
+                throw OutOfMemoryException();
+            count+=countRight;
+        }
     }
     return count;
 }