fortran: Fix conv of UNION constructors [PR105310]
This fixes an ICE when a UNION is the (1+8*2^n)-th field in a DEC
STRUCTURE when compiled with -finit-derived -finit-local-zero.
The problem was CONSTRUCTOR_APPEND_ELT from within gfc_conv_union_initializer
modified the vector pointer, but the pointer was passed by-value,
so the old pointer from the caller (gfc_conv_structure) pointed to freed
memory.
PR fortran/105310
gcc/fortran/ChangeLog:
* trans-expr.cc (gfc_conv_union_initializer): Pass vec* by reference.
gcc/testsuite/ChangeLog:
* gfortran.dg/dec_union_12.f90: New test.