log: clear thread-local stream's ios flags on reuse

log: clear thread-local stream's ios flags on reuse

when reusing a thread-local std::ostream for a new log entry, clear any
ios flags (ie eofbit/failbit/badbit) that may have been set during its
previous use

without this, any stream errors (for example, passing a nullptr to
operator<< sets the badbit) will result in all log output on that thread
to be blank for the remainder of the process

Signed-off-by: Casey Bodley <cbodley@redhat.com>