Attempt to prevent double from of sqlite3_stmt

Report: #632

android-database-sqlcipher/src/main/java/net/sqlcipher/database/SQLiteCompiledSql.java

@@ -93,7 +93,7 @@ private void compile(String sql, boolean forceCompilation) {
         }
     }
 
-    /* package */ void releaseSqlStatement() {
+    /* package */ synchronized void releaseSqlStatement() {
         // Note that native_finalize() checks to make sure that nStatement is
         // non-null before destroying it.
         if (nStatement != 0) {