ipa: Fix the size of the streamid array used in the proxy code
ipa: Fix the size of the streamid array used in the proxy code

We want to store stream ids 0 to 0xff. This means we need to have
256 entries in this array.

Fixes: Coverity CID 1040697, CID 1040696, CID 1040695, CID 1040694

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis,telenoobie/libosmo-abis,telenoobie/libosmo-abis