Rather than crashing in match_results::format() when a reference to a marked subexpression is out of range, format the subexpression as empty (i.e., replace it with an empty string). Note that match_results::operator[]() has a range-check and returns a null match in this case, so this just re-uses that logic. git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@259682 91177308-0d34-0410-b5e6-96231b3b80d8