SQLFreeStmt(stmt,SQL_DROP) now returns an error when it's conn has been disconnected. (#113)

* SQLFreeStmt(stmt,SQL_DROP) now returns an error before trying to free statement resources when its connection already has been closed.

A crash occurred when the PostgreSQL server terminated and a connection was
subsequently attempted from an application when connection pooling by the
driver manager was enabled and several pooled connections existed.
In such cases, an invalid memory access would occur in ENTER_CONN_CS(conn)
in SQLFreeStmt(), or an invalid address free() would be attempted in
SC_clear_error() via PGAPI_FreeStmt().

"CC_cleanup()" doing "conn->status = CONN_NOT_CONNECTED" releases all
statements belonging to the connection, so this change will probably not cause
a resource leak.

* Make 4dc08c8 more robust.

After continued testing, I found that the earlier fix alone caused
crashes in similar attempts, so we improved the fix.

Author: harukat

odbcapi.c

@@ -399,8 +399,12 @@ SQLFreeStmt(HSTMT StatementHandle,
 		if (Option == SQL_DROP)
 		{
 			conn = stmt->hdbc;
+			if (!conn || (conn->status != CONN_CONNECTED && conn->status != CONN_EXECUTING))
+				return SQL_INVALID_HANDLE;
 			if (conn)
 				ENTER_CONN_CS(conn);
+			if (!conn || (conn->status != CONN_CONNECTED && conn->status != CONN_EXECUTING))
+				return SQL_INVALID_HANDLE;
 		}
 		else
 			ENTER_STMT_CS(stmt);