Add exception support in `Range#first`.

Closes: https://github.com/ruby/ruby/pull/2163

range.c

@@ -1012,6 +1012,9 @@ range_first(int argc, VALUE *argv, VALUE range)
 {
     VALUE n, ary[2];
 
+    if (NIL_P(RANGE_BEG(range))) {
+        rb_raise(rb_eRangeError, "cannot get the first element of beginless range");
+    }
     if (argc == 0) return RANGE_BEG(range);
 
     rb_scan_args(argc, argv, "1", &n);

test/ruby/test_range.rb

@@ -443,6 +443,8 @@ class TestRange < Test::Unit::TestCase
     assert_equal("a", ("a"..nil).first)
     assert_raise(RangeError) { (0..nil).last }
     assert_raise(RangeError) { (0..nil).last(3) }
+    assert_raise(RangeError) { (nil..0).first }
+    assert_raise(RangeError) { (nil..0).first(3) }
 
     assert_equal([0, 1, 2], (0..10).first(3.0))
     assert_equal([8, 9, 10], (0..10).last(3.0))