[flang] Fix crash from empty -DMACRO= (bug #64837)

[klausler]

Summary

Some vector indexing code in the preprocessor fails with empty
tokens or token sequences in predefined macros.

Fixes https://github.com/llvm/llvm-project/issues/64837.

[klausler]

Add regression test.

[PeteSteinfeld]

All builds and tests correctly and looks good.

flang/lib/Parser/preprocessor.cpp

for (; j < tokens; ++j) {
     Definition *def{&it->second};
     if (def->isDisabled()) {
       result.Put(input, j);
       continue;
     }
     if (!def->isFunctionLike()) {
       bool isRenaming{false};
       if (def->isPredefined()) {
-        std::string name{def->replacement().TokenAt(0).ToString()};
         std::string repl;
+        if (!def->replacement().empty()) {
+          std::string name{def->replacement().TokenAt(0).ToString()};
           if (name == "__FILE__") {
             repl = "\""s +
                 allSources_.GetPath(prescanner.GetCurrentProvenance()) + '"';
           } else if (name == "__LINE__") {
             std::string buf;
             llvm::raw_string_ostream ss{buf};
             ss << allSources_.GetLineNumber(prescanner.GetCurrentProvenance());
             repl = ss.str();
           }
+        }
         if (!repl.empty()) {
           ProvenanceRange insert{allSources_.AddCompilerInsertion(repl)};
           ProvenanceRange call{allSources_.AddMacroCall(
               insert, input.GetTokenProvenanceRange(j), repl)};
           result.Put(repl, call.start());
           continue;
         }
       }

flang/lib/Parser/token-sequence.h

public:
   void clear();
   void pop_back();
   void shrink_to_fit();
   void swap(TokenSequence &);
 
   std::size_t SizeInTokens() const { return start_.size(); }
   std::size_t SizeInChars() const { return char_.size(); }
 
-  CharBlock ToCharBlock() const { return {&char_[0], char_.size()}; }
+  CharBlock ToCharBlock() const {
+    return char_.empty() ? CharBlock{} : CharBlock{&char_[0], char_.size()};
+  }
   std::string ToString() const { return ToCharBlock().ToString(); }
 
   CharBlock TokenAt(std::size_t token) const {
-    return {&char_[start_.at(token)], TokenBytes(token)};
+    if (auto bytes{TokenBytes(token)}) {
+      return {&char_[start_.at(token)], bytes};
+    } else { // char_ could be empty
+      return {};
+    }
   }
   char CharAt(std::size_t j) const { return char_.at(j); }
   CharBlock CurrentOpenToken() const {
     return {&char_[nextStart_], char_.size() - nextStart_};
   }
 
   std::size_t SkipBlanks(std::size_t) const;