re.results.form: Format out-of-range subexpression references as null

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.

llvm-svn: 259682
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f42ef3efcad9095920232a0a710a80c5c4720170
diff --git a/include/regex b/include/regex
index ca455f0..f1f3264 100644
--- a/include/regex
+++ b/include/regex
@@ -5387,8 +5387,8 @@
                 if ('0' <= *__fmt_first && *__fmt_first <= '9')
                 {
                     size_t __i = *__fmt_first - '0';
-                    __out = _VSTD::copy(__matches_[__i].first,
-                                       __matches_[__i].second, __out);
+                    __out = _VSTD::copy((*this)[__i].first,
+                                        (*this)[__i].second, __out);
                 }
                 else
                 {
@@ -5439,8 +5439,8 @@
                             ++__fmt_first;
                             __i = 10 * __i + *__fmt_first - '0';
                         }
-                        __out = _VSTD::copy(__matches_[__i].first,
-                                           __matches_[__i].second, __out);
+                        __out = _VSTD::copy((*this)[__i].first,
+                                            (*this)[__i].second, __out);
                     }
                     else
                     {