[Patch] Fortran: add contiguous check for ptr assignment, fix non-contig check (PR97242)

[Tobias Burnus]

The non-contiguous had both check false positive and false
negative results. Some more refinements
are surely possible, but hopefully there are no longer
false positives.

I also now used this check for pointer assignments where the
LHS pointer has the contiguous attribute.

In the non-contiguous-check function:
- for 'dt(i)%array' it returned true due to dt(i) but that's
   an element, which is contiguous.
- ref_size (which is a size) is compared with 'arr_size' calculated
   via dep_difference,, which returns upper-lower but array size is
   (upper-lower)+1.
- fixed a memory leak.

OK?

Tobias

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