range.c: fixed a begin-less ranges issue.

src/range.c

@@ -409,7 +409,7 @@ mrb_range_beg_len(mrb_state *mrb, mrb_value range, mrb_int *begp, mrb_int *lenp,
   if (!mrb_range_p(range)) return MRB_RANGE_TYPE_MISMATCH;
   r = mrb_range_ptr(mrb, range);
 
-  beg = mrb_int(mrb, RANGE_BEG(r));
+  beg = mrb_nil_p(RANGE_BEG(r)) ? 0 : mrb_int(mrb, RANGE_BEG(r));
   end = mrb_nil_p(RANGE_END(r)) ? -1 : mrb_int(mrb, RANGE_END(r));
   excl = mrb_nil_p(RANGE_END(r)) ? 0 : RANGE_EXCL(r);
 

test/t/array.rb

@@ -58,6 +58,7 @@ assert('Array#[]', '15.2.12.5.4') do
   assert_equal(["b", "c"], a[1,2])
   assert_equal(["b", "c", "d"], a[1..-2])
   assert_equal(["b", "c", "d", "e"], a[1..])
+  assert_equal(["a", "b", "c"], a[..2])
   skip unless Object.const_defined?(:Float)
   assert_equal("b", a[1.1])
 end