say: check size before write

We potentially pass negative size to safe_write in say_default if total
is negative. We can't hit this issue currently because formatting
functions currently do not return -1. Yet let's check total value where
appropriate.

By the way fix restoring errno in log_vsay. format_log_entry_full is
introduced for this purpuse as SNPRINT macro returns in case of error.

Also add assertion to safe_write.

This patch addresses coverity complain 1527154
(https://scan7.scan.coverity.com/reports.htm#v43693/p13437/fileInstanceId=129123623&defectInstanceId=18608452&mergedDefectId=1527154)

Part of #7913

NO_DOC=code health
NO_CHANGELOG=code health
NO_TEST=code health