fxt: set ptr to NULL to avoid double free

3 jobs for !34 with fxt_double_free in 21 minutes and 26 seconds (queued for 7 seconds)
latest detached
Status Name Job ID Coverage
  Build
passed build #1502938

00:06:13

 
  Deploy
passed check_linux #1502939
linux

00:11:01

passed simgrid #1502940

00:04:11