[flang] Fix new assertion failure

[klausler]

Summary

Don't compute the address of element [0] of a vector when the vector
is empty, even if the address is not used.

[clementval]

LGTM

flang/lib/Parser/token-sequence.cpp

if (TokenAt(j).FirstNonBlank() == '!') {
         return *this;
       }
     }
   }
   return *this;
 }
 
 void TokenSequence::Emit(CookedSource &cooked) const {
-  cooked.Put(&char_[0], char_.size());
+  if (auto n{char_.size()}) {
+    cooked.Put(&char_[0], n);
     cooked.PutProvenanceMappings(provenances_);
   }
+}
 
 llvm::raw_ostream &TokenSequence::Dump(llvm::raw_ostream &o) const {
   o << "TokenSequence has " << char_.size() << " chars; nextStart_ "
     << nextStart_ << '\n';
   for (std::size_t j{0}; j < start_.size(); ++j) {
     o << '[' << j << "] @ " << start_[j] << " '" << TokenAt(j).ToString()
       << "'\n";
   }