fxt: set ptr to NULL to avoid double free

3 jobs for !34 with fxt_double_free in 20 minutes and 32 seconds (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Build
passed #1467879
build

00:05:44

 
  Deploy
passed #1467880
linux
check_linux

00:10:47

passed #1467881
simgrid

00:04:00