No NULL check in mgos_rpc_call_oplya()

[e3b0c442]

Hello.

There is an inconsistency in the mgos_ api versus the documented mg_ API for sending RPC messages. mg_ API docs explicitly state that the callback and callback args can be NULL for messages where no response is expected. However, in the mgos_ API does not check for NULL callbacks and therefore attempts to call a null function, causing a system crash. I have verified that adding a nop callback function in place of NULL in mgos_rpc_send() resolves the crashing issues.

This could be remedied in one of two ways based on my examination of the code:

• mgos_rpc_call_oplya()(https://github.com/mongoose-os-libs/rpc-common/blob/master/src/mgos_rpc.c#L574-L603) can check for NULL in oplya_arg->cb before attempting to call the function,

or,

• modify mgos_rpc_call() (https://github.com/mongoose-os-libs/rpc-common/blob/master/src/mgos_rpc.c#L605-L620) to check for NULL and adjust the args for the mg_rpc_callf() call accordingly.

I consider the second to be preferable to avoid unnecessary heap allocation.

Please let me know if you need more information.