Free err strings correctly

src/rocksdb.rs

@@ -163,8 +163,11 @@ pub trait Writable {
 
 fn error_message(ptr: *const i8) -> String {
     let c_str = unsafe { CStr::from_ptr(ptr) };
-//TODO I think we're leaking the c string here; should be a call to free once realloced into rust String
-    from_utf8(c_str.to_bytes()).unwrap().to_owned()
+    let s = from_utf8(c_str.to_bytes()).unwrap().to_owned();
+    unsafe{
+        libc::free(ptr as *mut libc::c_void);
+    }
+    s
 }
 
 impl RocksDB {