PVS-Studio warning V614 for Firebird

void blb::move(....)
{
  ....
  const FieldNode* fieldNode;
  if (field)
  {
    if ((fieldNode = ExprNode::as<FieldNode>(field)))
    ....
  }
  ....
  const USHORT id = fieldNode->fieldId;
  ....
}

This suspicious code was found in Firebird project by PVS-Studio static code analyzer.
Warning message is:
V614 Potentially uninitialized pointer 'fieldNode' used. blb.cpp 1043

PVS-Studio is a static analyzer for detecting bugs in the source code of applications written in C, C++, C++11, C++/CX. Site: http://www.viva64.com/en/pvs-studio/