Fix negative Bignum conversion

Introduced in 4792a91

`rb_absint_size` return the number of bytes needed to fit
the absolute integer, but negative integers need the sign, so one more
bit, and potentially one more byte.

Author: byroot

ext/bigdecimal/bigdecimal.c

@@ -2772,6 +2772,9 @@ rb_big_convert_to_BigDecimal(VALUE val, RB_UNUSED_VAR(size_t digs), int raise_ex
 
     size_t size = rb_absint_size(val, NULL);
     int sign = FIX2INT(rb_big_cmp(val, INT2FIX(0)));
+    if (sign < 0) {
+        size += 1;
+    }
     if (size <= sizeof(long)) {
         if (sign < 0) {
             return rb_int64_convert_to_BigDecimal(NUM2LONG(val), digs, raise_exception);

test/bigdecimal/test_bigdecimal.rb

@@ -2089,6 +2089,16 @@ def test_initialize_copy_dup_clone_frozen_error
     assert_raise(err) { bd.send(:initialize_dup, bd2) }
   end
 
+  def test_llong_min
+    # https://github.com/ruby/bigdecimal/issues/199
+    # Between LLONG_MIN and -ULLONG_MAX
+    llong_min = -(2 ** 63 + 1)
+    assert_equal BigDecimal(llong_min.to_s), BigDecimal(llong_min)
+
+    minus_ullong_max = -(2 ** 64 - 1)
+    assert_equal BigDecimal(minus_ullong_max.to_s), BigDecimal(minus_ullong_max)
+  end
+
   def assert_no_memory_leak(code, *rest, **opt)
     code = "8.times {20_000.times {begin #{code}; rescue NoMemoryError; end}; GC.start}"
     super(["-rbigdecimal"],