[Bug #21680] Fix (base**power_of_two).digits(base) bug (ruby#15144)

Fix wrong condition in base multiplying loop.

Author: tompng

numeric.c

@@ -5553,7 +5553,7 @@ rb_int_digits_bigbase(VALUE num, VALUE base)
     }
 
     bases = rb_ary_new();
-    for (VALUE b = base; int_lt(b, num) == Qtrue; b = rb_int_mul(b, b)) {
+    for (VALUE b = base; int_le(b, num) == Qtrue; b = rb_int_mul(b, b)) {
         rb_ary_push(bases, b);
     }
     digits = rb_ary_new_from_args(1, num);

test/ruby/test_bignum.rb

@@ -821,6 +821,9 @@ def test_digits
     assert_equal([7215, 2413, 6242], T1024P.digits(10_000).first(3))
     assert_equal([11], 11.digits(T1024P))
     assert_equal([T1024P - 1, 1], (T1024P + T1024P - 1).digits(T1024P))
+    bug21680 = '[ruby-core:123769] [Bug #21680]'
+    assert_equal([0] * 64 + [1], (2**512).digits(256), bug21680)
+    assert_equal([0] * 128 + [1], (123**128).digits(123), bug21680)
   end
 
   def test_digits_for_negative_numbers