compiler: fix off-by-1 array type len in Type::gc_symbol_constructor

compiler: fix off-by-1 array type len in Type::gc_symbol_constructor

    Array type being built to hold GC var initializer was being created
    with an extra/unneeded slot. Fix up the code to insure that the array
    length matches the length of the initializer list.

    Reviewed-on: https://go-review.googlesource.com/34413

From-SVN: r243731