[flang][runtime] MayAlias() must be false for Unallocated descriptors

[klausler]

Summary

When either descriptor argument to MayAlias() is not allocated, the
result must be false by definition, since the values of the extents
in the dimensional information may be uninitialized.

flang/runtime/assign.cpp

static inline bool RangesOverlap(const char *aStart, const char *aEnd,
     const char *bStart, const char *bEnd) {
   return aEnd >= bStart && bEnd >= aStart;
 }
 
 // Predicate: could the left-hand and right-hand sides of the assignment
 // possibly overlap in memory?  Note that the descriptors themeselves
 // are included in the test.
 static bool MayAlias(const Descriptor &x, const Descriptor &y) {
+  const char *xBase{x.OffsetElement()};
+  const char *yBase{y.OffsetElement()};
+  if (!xBase || !yBase) {
+    return false; // not both allocated
+  }
   const char *xDesc{reinterpret_cast<const char *>(&x)};
   const char *xDescLast{xDesc + x.SizeInBytes()};
   const char *yDesc{reinterpret_cast<const char *>(&y)};
   const char *yDescLast{yDesc + y.SizeInBytes()};
   std::int64_t xLeast, xMost, yLeast, yMost;
   MaximalByteOffsetRange(x, xLeast, xMost);
   MaximalByteOffsetRange(y, yLeast, yMost);
-  const char *xBase{x.OffsetElement()};
-  const char *yBase{y.OffsetElement()};
   if (RangesOverlap(xDesc, xDescLast, yBase + yLeast, yBase + yMost) ||
       RangesOverlap(yDesc, yDescLast, xBase + xLeast, xBase + xMost)) {
     // A descriptor overlaps with the storage described by the other;
     // this can arise when an allocatable or pointer component is
     // being assigned to/from.
     return true;
   }
   if (!RangesOverlap(