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.