Add fuzzer for FDE CSS syntax parser.

xfa/fxfa/parser/xfa_basic_imp.cpp

Index: xfa/fxfa/parser/xfa_basic_imp.cpp
diff --git a/xfa/fxfa/parser/xfa_basic_imp.cpp b/xfa/fxfa/parser/xfa_basic_imp.cpp
index 86a96bbd63db6e807ce8ee8c4a32090daa288402..f7c2606501bd526a159ef3a0942a0cb8dacd64e2 100644
--- a/xfa/fxfa/parser/xfa_basic_imp.cpp
+++ b/xfa/fxfa/parser/xfa_basic_imp.cpp
@@ -557,9 +557,10 @@ int32_t CXFA_WideTextRead::ReadString(FX_WCHAR* pStr,
                                       int32_t iMaxLength,
                                       FX_BOOL& bEOS,
                                       int32_t const* pByteSize) {
-  if (iMaxLength > m_wsBuffer.GetLength() - m_iPosition) {
-    iMaxLength = m_wsBuffer.GetLength() - m_iPosition;
-  }
+  iMaxLength = std::min(iMaxLength, m_wsBuffer.GetLength() - m_iPosition);
+  if (iMaxLength == 0)
+    return 0;
+
   FXSYS_wcsncpy(pStr, m_wsBuffer.c_str() + m_iPosition, iMaxLength);
   m_iPosition += iMaxLength;
   bEOS = IsEOF();