Fix potential access-off-the-end-of-memory in varbit_out(): it fetched the

Fix potential access-off-the-end-of-memory in varbit_out(): it fetched the
byte after the last full byte of the bit array, regardless of whether that
byte was part of the valid data or not.  Found by buildfarm testing.
Thanks to Stefan Kaltenbrunner for nailing down the cause.