goprotobuf: Fix bug in GetExtensions.

It is documented to return nil when an extension is missing;
we weren't doing that.

LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://codereview.appspot.com/107230043
diff --git a/proto/extensions.go b/proto/extensions.go
index c40b392..e592053 100644
--- a/proto/extensions.go
+++ b/proto/extensions.go
@@ -300,6 +300,9 @@
 	extensions = make([]interface{}, len(es))
 	for i, e := range es {
 		extensions[i], err = GetExtension(epb, e)
+		if err == ErrMissingExtension {
+			err = nil
+		}
 		if err != nil {
 			return
 		}
diff --git a/proto/extensions_test.go b/proto/extensions_test.go
new file mode 100644
index 0000000..03f756b
--- /dev/null
+++ b/proto/extensions_test.go
@@ -0,0 +1,60 @@
+// Go support for Protocol Buffers - Google's data interchange format
+//
+// Copyright 2014 The Go Authors.  All rights reserved.
+// http://code.google.com/p/goprotobuf/
+//
+// Redistribution and use in source and binary forms, with or without
+// modification, are permitted provided that the following conditions are
+// met:
+//
+//     * Redistributions of source code must retain the above copyright
+// notice, this list of conditions and the following disclaimer.
+//     * Redistributions in binary form must reproduce the above
+// copyright notice, this list of conditions and the following disclaimer
+// in the documentation and/or other materials provided with the
+// distribution.
+//     * Neither the name of Google Inc. nor the names of its
+// contributors may be used to endorse or promote products derived from
+// this software without specific prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+// OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+package proto_test
+
+import (
+	"testing"
+
+	pb "./testdata"
+	"code.google.com/p/goprotobuf/proto"
+)
+
+func TestGetExtensionsWithMissingExtensions(t *testing.T) {
+	msg := &pb.MyMessage{}
+	ext1 := &pb.Ext{}
+	if err := proto.SetExtension(msg, pb.E_Ext_More, ext1); err != nil {
+		t.Fatalf("Could not set ext1: %s", ext1)
+	}
+	exts, err := proto.GetExtensions(msg, []*proto.ExtensionDesc{
+		pb.E_Ext_More,
+		pb.E_Ext_Text,
+	})
+	if err != nil {
+		t.Fatalf("GetExtensions() failed: %s", err)
+	}
+	if exts[0] != ext1 {
+		t.Errorf("ext1 not in returned extensions: %T %v", exts[0], exts[0])
+	}
+	if exts[1] != nil {
+		t.Errorf("ext2 in returned extensions: %T %v", exts[1], exts[1])
+	}
+}