input/protocoldata: Check if lookup_protocol_name is valid

input/protocoldata: Check if lookup_protocol_name is valid

As of today, the code does not crash because all protocols have names.
But the name of the first element is not checked to be non-NULL
before calling strcmp(). This can result in a future crash if the
code logic changes.

This is mostly a change to robustify the lookup_protocol_name() function.

Also, this commit simplifies the indexing logic of the protocols array.