Fix ary_make_partial_step for compaction

ary could change embeddedness due to compaction, so we should only get
the pointer after allocations.

The included test was crashing with:

    TestArray#test_slice_gc_compact_stress
    ruby/lib/pp.rb:192: [BUG] Segmentation fault at 0x0000000000000038

array.c

@@ -1231,12 +1231,13 @@ ary_make_partial_step(VALUE ary, VALUE klass, long offset, long len, long step)
     assert(offset+len <= RARRAY_LEN(ary));
     assert(step != 0);
 
-    const VALUE *values = RARRAY_CONST_PTR(ary);
     const long orig_len = len;
 
     if (step > 0 && step >= len) {
         VALUE result = ary_new(klass, 1);
         VALUE *ptr = (VALUE *)ARY_EMBED_PTR(result);
+        const VALUE *values = RARRAY_CONST_PTR(ary);
+
         RB_OBJ_WRITE(result, ptr, values[offset]);
         ARY_SET_EMBED_LEN(result, 1);
         return result;
@@ -1254,13 +1255,17 @@ ary_make_partial_step(VALUE ary, VALUE klass, long offset, long len, long step)
     VALUE result = ary_new(klass, len);
     if (ARY_EMBED_P(result)) {
         VALUE *ptr = (VALUE *)ARY_EMBED_PTR(result);
+        const VALUE *values = RARRAY_CONST_PTR(ary);
+
         for (i = 0; i < len; ++i) {
             RB_OBJ_WRITE(result, ptr+i, values[j]);
             j += step;
         }
         ARY_SET_EMBED_LEN(result, len);
     }
     else {
+        const VALUE *values = RARRAY_CONST_PTR(ary);
+
         RARRAY_PTR_USE(result, ptr, {
             for (i = 0; i < len; ++i) {
                 RB_OBJ_WRITE(result, ptr+i, values[j]);

test/ruby/test_array.rb

@@ -1704,6 +1704,10 @@ def test_slice_out_of_range
 
   def test_slice_gc_compact_stress
     EnvUtil.under_gc_compact_stress { assert_equal([1, 2, 3, 4, 5], (0..10).to_a[1, 5]) }
+    EnvUtil.under_gc_compact_stress do
+      a = [0, 1, 2, 3, 4, 5]
+      assert_equal([2, 1, 0], a.slice((2..).step(-1)))
+    end
   end
 
   def test_slice!