[flang] Set "undefined" NEXTREC=n variable to 0 rather than random garbage

[klausler]

Summary

12.10.2.17 defines that a INQUIRE statement's NEXTREC=n output value
for a unit that is not connected for direct access becomes undefined,
but the current I/O runtime can fail in a confusing manner by trying
to return uninitialized stack garbage.

Reported on Slack by Tarun Prabhu as an intermittent failure in
the gfortran regression test inquire_pre.f90.

[tarunprabhu]

LGTM. Thanks for fixing this.

[everythingfunctional]

Is there an equivalent to -finit-integer=99999? If so, I think it would be preferable if this initialisation for an "undefined" variable respected that, in order to help when looking for uses of undefined variables, since referencing an undefined variable makes it invalid Fortran.

flang/runtime/io-api.cpp

 bool IONAME(InquirePendingId)(Cookie cookie, std::int64_t id, bool &result) {
   IoStatementState &io{*cookie};
   return io.Inquire(HashInquiryKeyword("PENDING"), id, result);
 }
 
 bool IONAME(InquireInteger64)(
     Cookie cookie, InquiryKeywordHash inquiry, std::int64_t &result, int kind) {
   IoStatementState &io{*cookie};
-  std::int64_t n;
+  std::int64_t n{0}; // safe "undefined" value
   if (io.Inquire(inquiry, n)) {
     if (SetInteger(result, kind, n)) {
       return true;
     }
     io.GetIoErrorHandler().SignalError(
         "InquireInteger64(): bad INTEGER kind(%d) or out-of-range "
-        "value(%jd) "
-        "for result",
+        "value(%jd) for result",
         kind, static_cast<std::intmax_t>(n));
   }
   return false;
 }
 
 enum Iostat IONAME(EndIoStatement)(Cookie cookie) {
   IoStatementState &io{*cookie};
   return static_cast<enum Iostat>(io.EndIoStatement());