gtk: Remove "check NULL" comments

All of these are GLib/GTK functions that either succeed or crash out with a fatal error, but cannot return NULL. The version of g_malloc0() that can in theory return NULL (although usually not on Linux) is g_try_malloc0(), and GTK widget creation never survives out-of-memory conditions.

If cdebconf genuinely runs out of memory, there isn't really going to be anything we can do about it (popping up a dialog or writing to the syslog is certainly going to need memory!) so the realistic response is to just crash. GLib calls g_error() on out-of-memory, which has the least-bad feasible handling for OOM: it will do its best to write a message to stderr, and then abort() to leave a core dump.