Fixed IO#read with buf.

I hit the following two problems.

  - `io.read(0, buf)` always returned a new empty string object.
  - `io.read(num, buf)` was appending data to the given `buf`.
    This also meant that `buf` was never empty if EOF was reached.

mrbgems/mruby-io/src/io.c

@@ -1702,7 +1702,14 @@ io_read(mrb_state *mrb, mrb_value io)
         mrb_raisef(mrb, E_ARGUMENT_ERROR, "negative length %d given", length);
       }
       if (length == 0) {
-        return mrb_str_new(mrb, NULL, 0);
+        if (mrb_nil_p(outbuf)) {
+          outbuf = mrb_str_new(mrb, NULL, 0);
+        }
+        else {
+          mrb_str_modify(mrb, mrb_str_ptr(outbuf));
+          RSTR_SET_LEN(mrb_str_ptr(outbuf), 0);
+        }
+        return outbuf;
       }
     }
   }
@@ -1710,6 +1717,10 @@ io_read(mrb_state *mrb, mrb_value io)
   if (mrb_nil_p(outbuf)) {
     outbuf = mrb_str_new_capa(mrb, MRB_IO_BUF_SIZE);
   }
+  else {
+    mrb_str_modify(mrb, mrb_str_ptr(outbuf));
+    RSTR_SET_LEN(mrb_str_ptr(outbuf), 0);
+  }
   if (!length_given) {          /* read as much as possible */
     return io_read_all(mrb, fptr, outbuf);
   }

mrbgems/mruby-io/test/io.rb

@@ -150,6 +150,30 @@ assert "IO#read(n) with n > IO::BUF_SIZE" do
   end
 end
 
+assert "IO#read(n, buf)" do
+  IO.open(IO.sysopen($mrbtest_io_rfname)) do |io|
+    buf = "12345"
+    assert_same buf, io.read(0, buf)
+    assert_equal "", buf
+
+    buf = "12345"
+    assert_same buf, io.read(5, buf)
+    assert_equal "mruby", buf
+
+    buf = "12345"
+    assert_same buf, io.read(nil, buf)
+    assert_equal " io test\n", buf
+
+    buf = "12345"
+    assert_nil io.read(99, buf)
+    assert_equal "", buf
+
+    buf = "12345"
+    assert_same buf, io.read(0, buf)
+    assert_equal "", buf
+  end
+end
+
 assert('IO#readchar', '15.2.20.5.15') do
   # almost same as IO#getc
   IO.open(IO.sysopen($mrbtest_io_rfname)) do |io|