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

…rbage

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.

Differential Revision: https://reviews.llvm.org/D152295

flang/runtime/io-api.cpp

@@ -1451,15 +1451,14 @@ bool IONAME(InquirePendingId)(Cookie cookie, std::int64_t id, bool &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;