[flang] Fix potential segfault in PointerAssociateRemapping

[clementval]

Summary

The bounds descriptor passed to the function is an array of [2, newRank]
size. Update the code so the rank is retrieved from the second dimension
and not the rank of the descriptor directly as it will be 2 in any case.

[clementval]

Add cast

[jeanPerier]

LGTM

flang/runtime/pointer.cpp

 void RTNAME(PointerAssociateRemapping)(Descriptor &pointer,
     const Descriptor &target, const Descriptor &bounds, const char *sourceFile,
     int sourceLine) {
   pointer = target;
   pointer.raw().attribute = CFI_attribute_pointer;
   Terminator terminator{sourceFile, sourceLine};
   SubscriptValue byteStride{/*captured from first dimension*/};
   std::size_t boundElementBytes{bounds.ElementBytes()};
-  pointer.raw().rank = bounds.rank();
-  for (int j{0}; j < bounds.rank(); ++j) {
+  std::size_t boundsRank{
+      static_cast<std::size_t>(bounds.GetDimension(1).Extent())};
+  pointer.raw().rank = boundsRank;
+  for (int j{0}; j < boundsRank; ++j) {
     auto &dim{pointer.GetDimension(j)};
     dim.SetBounds(GetInt64(bounds.ZeroBasedIndexedElement<const char>(2 * j),
                       boundElementBytes, terminator),
         GetInt64(bounds.ZeroBasedIndexedElement<const char>(2 * j + 1),
             boundElementBytes, terminator));
     if (j == 0) {
       byteStride = dim.ByteStride() * dim.Extent();
     } else {