Activate option "MEM_USE_POOLS_TRY_BIGGER_POOL"

The lwip stack uses different memory pools that are used to send and receive frames. Each pool holds memory entries of a specific size.
In case a frame shall be sent, the stack looks for a pool that fits best to the data size of the frame to be sent. When the pool was determined the stack then tries to get an element of that pool in order to put in the data that shall be sent. Now, if there is no element left in the pool that can be used, the stack currently runs into an error and does not send the frame.

Therefore the option "MEM_USE_POOLS_TRY_BIGGER_POOL" shall be activated. So in case there is no element left in the pool the next bigger pool shall be taken into account.