Silence compiler warnings about unused msg_truncated variables.

Silence compiler warnings about unused msg_truncated variables.

BTW, the error and notice handling could be made much smarter wrt.
truncation. The caller of handle_notice/error_message knows the message size
in advance, so it could easily pass it down and we could allocate a buffer
of the right size, instead of using a fixed size buffer that can be too
small.