Fix float-approx negative left shift UB

Reported by toto.
4 jobs for master in 8 minutes and 23 seconds (queued for 1 minute and 24 seconds)
latest
Status Job ID Name Coverage
  Build
passed autoconf #11159
docker

00:04:28

passed cmake #11160
docker

00:02:17

passed meson #11161
docker

00:01:47

 
  Test
passed whitespace #11162
docker

00:00:05