Range-check the integral Rational offset

Every branch of offset_to_sec range-checks the resulting number of
seconds except one path through the Rational branch: when the day
fraction is an integral Rational, n is assigned inside the if arm and
reaches *rof without passing the guard that sits in the else arm.

DateTime.new(2024, 1, 1, 0, 0, 0, Rational(2, 1)) therefore produced a
48-hour offset, while the equivalent Integer 2 is rejected and falls
back to +00:00. Rational(49710, 1) is 4_294_944_000 seconds, over
INT_MAX, so the (int) narrowing turned a large positive offset into a
negative one.

Move the check below the if/else so it covers both arms. That also
bounds n before the narrowing. Rational(1, 1) is exactly
DAY_IN_SECONDS and the guard is inclusive, so in-range values are
unaffected.

Author: sjh9714

ext/date/date_core.c

@@ -2621,9 +2621,9 @@ offset_to_sec(VALUE vof, int *rof)
 		if (!FIXNUM_P(vn))
 		    return 0;
 		n = FIX2LONG(vn);
-		if (n < -DAY_IN_SECONDS || n > DAY_IN_SECONDS)
-		    return 0;
 	    }
+	    if (n < -DAY_IN_SECONDS || n > DAY_IN_SECONDS)
+		return 0;
 	    *rof = (int)n;
 	    return 1;
 	}

test/date/test_date_new.rb

@@ -192,6 +192,21 @@ def test_civil__ex
     end
   end
 
+  def test_civil__offset
+    d = DateTime.civil(2001,2,3, 0,0,0, Rational(1, 1))
+    assert_equal(1.to_r, d.offset)
+    d = DateTime.civil(2001,2,3, 0,0,0, Rational(-1, 1))
+    assert_equal(-1.to_r, d.offset)
+
+    # An out-of-range offset is ignored, as it is for the equivalent Integer.
+    d = DateTime.civil(2001,2,3, 0,0,0, 2)
+    assert_equal(0, d.offset)
+    d = DateTime.civil(2001,2,3, 0,0,0, Rational(2, 1))
+    assert_equal(0, d.offset)
+    d = DateTime.civil(2001,2,3, 0,0,0, Rational(49710, 1))
+    assert_equal(0, d.offset)
+  end
+
   def test_civil__reform
     d = Date.jd(Date::ENGLAND, Date::ENGLAND)
     dt = DateTime.jd(Date::ENGLAND, 0,0,0,0, Date::ENGLAND)