Fix Undefined Behavior in MLIR Diagnostic: don't call memcpy with a nullptr source

[mehdi_amini]

Summary

This happens when streaming an empty Twine as part of a diagnostic.

mlir/lib/IR/Diagnostics.cpp

 
 /// Convert a Twine to a StringRef. Memory used for generating the StringRef is
 /// stored in 'strings'.
 static StringRef twineToStrRef(const Twine &val,
                                std::vector<std::unique_ptr<char[]>> &strings) {
   // Allocate memory to hold this string.
   SmallString<64> data;
   auto strRef = val.toStringRef(data);
+  if (strRef.empty())
+    return strRef;
+
   strings.push_back(std::unique_ptr<char[]>(new char[strRef.size()]));
   memcpy(&strings.back()[0], strRef.data(), strRef.size());
-
   // Return a reference to the new string.
   return StringRef(&strings.back()[0], strRef.size());
 }
 
 /// Stream in a Twine argument.
 Diagnostic &Diagnostic::operator<<(char val) { return *this << Twine(val); }
 Diagnostic &Diagnostic::operator<<(const Twine &val) {
   arguments.push_back(DiagnosticArgument(twineToStrRef(val, strings)));