[gfortran,patch] Fix PR33636, EXPR_ARRAY isn't necessarily a constant array

[François-Xavier Coudert]

> Ok, but please also add a testcase.

Sorry, I should have said this: this fixes the current testsuite
failure for gfortran.dg/derived_comp_array_ref_5.f90. Thus, I don't
think we need another testcase.

FX