[cmyth] Update bad error handling

connection.c:

- Check NULL connection before, not after
- Return consistent ERROR values
[cmyth] Update bad error handling

storagegroup.c:

- Return consistent ERROR values.
- Use NULL instead 0 for null pointer (Debug).
[cmyth] No SQL result for DML statement

- Use cmyth_mysql_query() instead cmyth_mysql_query_result() to avoid error message.
[cmyth] Keep LiveTV recording

- cmyth_mysql_keep_livetv_recording(): New function to change recording recgroup of liveTV recording.
- cmyth_proginfo_storagegroup(): Returns the storage group of recording.
- cmyth_proginfo_generate_pixmap(): Call backend command QUERY_GENPIXMAP2
[cmyth] GENERATED_PIXMAP breaks event connection stream

- cmyth_event_get_message() breaks on GENERATED_PIXMAP: Review "leftover data" and add comments.
- Refactor __cmyth_rcv_data(): It will be use to leftover any data.
- __cmyth_rcv_okay(): prefer to use __cmyth_rcv_data() instead cmyth_rcv_string() when  leftover data.