Ensure LOG* and CHECK* macros are statement-safe

[madscientist]

The LOG and LOGF macros had been modified to be statement-safe, but the
LOG_IF, CHECK, LOGF_IF, CHECKF, and CHECK_F macros were all still
unsafe in face of code where single-statement blocks were not enclosed
in {}.

For example code like this:

  if (!foobar)
      CHECKF(goodness, "badness detected!");
  else
      handle_foobar(foobar);

would fail in subtle and possibly dangerous ways.

Fix this by combining multiple if-statements into a single conditional
and inverting the conditions, then adding an empty then-block and moving
the log statement to the else-block.

[KjellKod]

Looks good. Merge is pending testing.

[KjellKod]

Nice work, thanks!