[flang] Fix a crash when cosubscript list is empty

[PeteSteinfeld]

Summary

When there are errors in the evaluation of every cosubscript expression in a
coindexed object, the compiler would crash. I fixed this by just checking to
see if there were errors in the evaluation of the cosubscripts before
constructing the DataRef for the coindexed object.

flang/lib/Semantics/expression.cpp

if (cosubsOk && !reversed.empty()) {
         Say("'%s' has corank %d, but coindexed reference has %d cosubscripts"_err_en_US,
             symbol.name(), symbol.Corank(), numCosubscripts);
       }
     }
     for (const auto &imageSelSpec :
         std::get<std::list<parser::ImageSelectorSpec>>(x.imageSelector.t)) {
       std::visit(
           common::visitors{
-              [&](const auto &x) {Analyze(x.v); },
+              [&](const auto &x) { Analyze(x.v); },
           },
           imageSelSpec.u);
     }
     // Reverse the chain of symbols so that the base is first and coarray
     // ultimate component is last.
+    if (cosubsOk) {
       return Designate(
           DataRef{CoarrayRef{SymbolVector{reversed.crbegin(), reversed.crend()},
               std::move(subscripts), std::move(cosubscripts)}});
     }
+  }
   return std::nullopt;
 }
 
 int ExpressionAnalyzer::IntegerTypeSpecKind(
     const parser::IntegerTypeSpec &spec) {
   Expr<SubscriptInteger> value{
       AnalyzeKindSelector(TypeCategory::Integer, spec.v)};
   if (auto kind{ToInt64(value)}) {

flang/test/Semantics/resolve94.f90

 ! RUN: %S/test_errors.sh %s %t %f18
 ! C929   No specifier shall appear more than once in a given 
 !   image-selector-spec-list.
 ! C930 TEAM and TEAM_NUMBER shall not both appear in the same
 !   image-selector-spec-list.
 ! C931 A stat-variable in an image-selector shall not be a coindexed object.
 subroutine s1()
   use ISO_FORTRAN_ENV
   type(team_type) :: team1, team2
   real :: rCoarray[10,20,*]
   real :: rVar1, rVar2
   integer :: iVar1, iVar2
   integer, dimension(4) :: intArray
   integer :: intScalarCoarray[*]
   integer :: intCoarray[3, 4, *]
+  integer :: smallIntCoarray[4, *]
   intCoVar = 343
   ! OK
   rVar1 = rCoarray[1,2,3]
   !ERROR: 'rcoarray' has corank 3, but coindexed reference has 2 cosubscripts
   rVar1 = rCoarray[1,2]
   !ERROR: Must have INTEGER type, but is REAL(4)
   rVar1 = rCoarray[1,2,3.4]
+  !ERROR: Must have INTEGER type, but is REAL(4)
+  iVar1 = smallIntCoarray[3.4]
   !ERROR: Must be a scalar value, but is a rank-1 array
   rVar1 = rCoarray[1,intArray,3]
   ! OK
   rVar1 = rCoarray[1,2,3,STAT=iVar1, TEAM=team2]
   !ERROR: Team value must be of type TEAM_TYPE from module ISO_FORTRAN_ENV
   rVar1 = rCoarray[1,2,3,STAT=iVar1, TEAM=2]
   ! OK
   rVar1 = rCoarray[1,2,3,STAT=iVar1, TEAM_NUMBER=38]