[flang] Fixed a crash in CheckReduce() (#156382)

Added extra checks to fix the crash.

Fixes #156167

Author: eugeneepshteyn

flang/lib/Semantics/check-call.cpp

@@ -1985,8 +1985,9 @@ static void CheckReduce(
       }
     }
   }
-  const auto *result{
-      procChars ? procChars->functionResult->GetTypeAndShape() : nullptr};
+  const auto *result{procChars && procChars->functionResult
+          ? procChars->functionResult->GetTypeAndShape()
+          : nullptr};
   if (!procChars || !procChars->IsPure() ||
       procChars->dummyArguments.size() != 2 || !procChars->functionResult) {
     messages.Say(

flang/test/Semantics/reduce01.f90

@@ -71,6 +71,8 @@ pure function f11(x,y) result(res)
     type(pdt(max(x%len, y%len))) :: res
     res%ch = x%ch // y%ch
   end function
+  subroutine bad_reduce
+  end subroutine
 
   subroutine errors
     real :: a(10,10), b, c(10)
@@ -115,6 +117,8 @@ subroutine errors
     !ERROR: MASK= has no .TRUE. element, so IDENTITY= must be present
     b = reduce(a, f, reshape([(j > 100, j=1, 100)], shape(a)))
     b = reduce(a, f, reshape([(j == 50, j=1, 100)], shape(a))) ! ok
+    !ERROR: OPERATION= argument of REDUCE() must be a pure function of two data arguments
+    b = reduce(a, bad_reduce)
   end subroutine
   subroutine not_errors
     type(pdt(10)) :: a(10), b