Test using cgocheck=2 and fix compaction_filter test for that

.travis.yml

@@ -25,7 +25,7 @@ install:
   - go get -t ./...
 
 script:
-  - go test -v ./
+  - GODEBUG=cgocheck=2 go test -v ./
 
 notifications:
   email:

compaction_filter_test.go

@@ -11,9 +11,12 @@ func TestCompactionFilter(t *testing.T) {
 	var (
 		changeKey    = []byte("change")
 		changeValOld = []byte("old")
-		changeValNew = []byte("new")
+		changeValNew = cBackedBytes([]byte("new"))
 		deleteKey    = []byte("delete")
 	)
+
+	defer freeCBackedBytes(changeValNew)
+
 	db := newTestDB(t, "TestCompactionFilter", func(opts *Options) {
 		opts.SetCompactionFilter(&mockCompactionFilter{
 			filter: func(level int, key, val []byte) (remove bool, newVal []byte) {

util.go

@@ -1,5 +1,6 @@
 package gorocksdb
 
+// #include <stdlib.h>
 import "C"
 import (
 	"reflect"
@@ -30,6 +31,19 @@ func charToByte(data *C.char, len C.size_t) []byte {
 	return value
 }
 
+// cBackedBytes returs a copy of the same byte slice which is backed by
+// malloced memory. This should be freed using freeCBackedBytes.
+func cBackedBytes(data []byte) []byte {
+	return charToByte(cByteSlice(data), C.size_t(len(data)))
+}
+
+// freeCBackedBytes frees a byte slice created by cBackedBytes
+func freeCBackedBytes(data []byte) {
+	sH := (*reflect.SliceHeader)(unsafe.Pointer(&data))
+	C.free(unsafe.Pointer(sH.Data))
+
+}
+
 // byteToChar returns *C.char from byte slice.
 func byteToChar(b []byte) *C.char {
 	var c *C.char