Build regex from channel and not sub->channel.

[patmarion]

The sub->channel is a shortened string clipped at ZCM_CHANNEL_MAXLEN=32 while channel is the full string.

[patmarion]

Hi @jbendes and @olsoni, this is a fix to a bug I encountered when using zcm-logger -c <channel_regex>. If the given regex string is longer than ZCM_CHANNEL_MAXLEN characters, which is only 32, then the string is clipped and the regex attempts to parse the clipped string and fails.

This change fixes the issue for me. But I do find the whole concept of clipping the string to be a bit sketchy here. It seems like maybe it should be a fatal error if someone calls subscribe with a channel name that is too long? I have an alternative fix where I introduce a --logger-regex command line arg that can accept a regex string and handle the channel name filtering in the logger's callback handler, rather than passing the regex string into the zcm subscriber channel.

[patmarion]

channel is the complete string. sub->channel has been truncated at ZCM_CHANNEL_MAXLEN. Note that just below here the sub is stored in a map using channel as the key.