[flang] Fix extent computation in finalization

[klausler]

Summary

The code that computed the extent of a dimension of a
non-allocatable/non-automatic component array during
finalization had a reversed subtraction; fix, and
use variables to make the code a little more readable.

[klausler]

ping

[jeanPerier]

LGTM

flang/runtime/derived.cpp

if (comp.genre() == typeInfo::Component::Genre::Allocatable ||
           }
         }
       }
     } else if (comp.genre() == typeInfo::Component::Genre::Data &&
         comp.derivedType() && !comp.derivedType()->noFinalizationNeeded()) {
       SubscriptValue extent[maxRank];
       const typeInfo::Value *bounds{comp.bounds()};
       for (int dim{0}; dim < comp.rank(); ++dim) {
-        extent[dim] = bounds[2 * dim].GetValue(&descriptor).value_or(0) -
-            bounds[2 * dim + 1].GetValue(&descriptor).value_or(0) + 1;
+        SubscriptValue lb{bounds[2 * dim].GetValue(&descriptor).value_or(0)};
+        SubscriptValue ub{
+            bounds[2 * dim + 1].GetValue(&descriptor).value_or(0)};
+        extent[dim] = ub >= lb ? ub - lb + 1 : 0;
       }
       StaticDescriptor<maxRank, true, 0> staticDescriptor;
       Descriptor &compDesc{staticDescriptor.descriptor()};
       const typeInfo::DerivedType &compType{*comp.derivedType()};
       for (std::size_t j{0}; j < elements; ++j) {
         compDesc.Establish(compType,
             descriptor.OffsetElement<char>(j * byteStride + comp.offset()),
             comp.rank(), extent);