Initialize output parameters

[vitalybuka]

Summary

If the function returns true, it should
set all output paremeters, similar to Output::preflightElement, or we
have UB on code like:

void *SaveInfo;
if (io.preflightFlowElement(i, SaveInfo))
  io.postflightFlowElement(SaveInfo);

It's going to be detected by msan with:
-Xclang -enable-noundef-analysis -mllvm -msan-eager-checks=1

llvm/lib/Support/YAMLTraits.cpp

unsigned Output::beginFlowSequence() {
   return 0;
 }
 
 void Output::endFlowSequence() {
   StateStack.pop_back();
   outputUpToEndOfLine(" ]");
 }
 
-bool Output::preflightFlowElement(unsigned, void *&) {
+bool Output::preflightFlowElement(unsigned, void *&SaveInfo) {
   if (NeedFlowSequenceComma)
     output(", ");
   if (WrapColumn && Column > WrapColumn) {
     output("\n");
     for (int i = 0; i < ColumnAtFlowStart; ++i)
       output(" ");
     Column = ColumnAtFlowStart;
     output("  ");
   }
+  SaveInfo = nullptr;
   return true;
 }
 
 void Output::postflightFlowElement(void *) {
   NeedFlowSequenceComma = true;
 }
 
 void Output::beginEnumScalar() {