Track connection with the statement resource

We don't want the sqlite3 connection getting garbage collected before
the statement is finalized.

Author: warmwaffles

c_src/sqlite3_nif.c

@@ -26,6 +26,7 @@ typedef struct connection
 
 typedef struct statement
 {
+    connection_t* conn;
     sqlite3_stmt* statement;
 } statement_t;
 
@@ -354,6 +355,9 @@ exqlite_prepare(ErlNifEnv* env, int argc, const ERL_NIF_TERM argv[])
         return make_error_tuple(env, "out_of_memory");
     }
 
+    enif_keep_resource(conn);
+    statement->conn = conn;
+
     rc = sqlite3_prepare_v3(conn->db, (char*)bin.data, bin.size, 0, &statement->statement, NULL);
     if (rc != SQLITE_OK) {
         enif_release_resource(statement);
@@ -865,6 +869,11 @@ statement_type_destructor(ErlNifEnv* env, void* arg)
         sqlite3_finalize(statement->statement);
         statement->statement = NULL;
     }
+
+    if (statement->conn) {
+        enif_release_resource(statement->conn);
+        statement->conn = NULL;
+    }
 }
 
 static int