[flang] Fixed a crash with undeclared variable in implicit-do loop (#149513)

Fixed a crash in the following example:
```
subroutine sub()
  implicit none
  print *, (i, i = 1, 2)  ! Problem: using undefined var in implied-do loop
end subroutine sub
```
The error message was already generated, but the compiler crashed before
it could display it.

Author: eugeneepshteyn

flang/lib/Semantics/check-do-forall.cpp

@@ -1180,7 +1180,9 @@ void DoForallChecker::Leave(const parser::IoControlSpec &ioControlSpec) {
 void DoForallChecker::Leave(const parser::OutputImpliedDo &outputImpliedDo) {
   const auto &control{std::get<parser::IoImpliedDoControl>(outputImpliedDo.t)};
   const parser::Name &name{control.name.thing.thing};
-  context_.CheckIndexVarRedefine(name.source, *name.symbol);
+  if (name.symbol) {
+    context_.CheckIndexVarRedefine(name.source, *name.symbol);
+  }
 }
 
 void DoForallChecker::Leave(const parser::StatVariable &statVariable) {

flang/test/Semantics/resolve40.f90

@@ -96,3 +96,10 @@ subroutine s12(x)
   !BECAUSE: 'x' is an INTENT(IN) dummy argument
   read(*,nml=nl)
 end
+
+subroutine s13()
+  implicit none
+  !ERROR: No explicit type declared for 'i'
+  !ERROR: No explicit type declared for 'i'
+  print *, (i, i = 1, 2)
+end