iso-dialog: Fix leak and quoting

Do not leak memory in case of task cancelled.
Quote "msg" in case it contains some no-xml character that could
came from translated strings.

Signed-off-by: Frediano Ziglio <[email protected]>
28 jobs for master in 34 minutes and 44 seconds (queued for 1 second)
Status Job ID Name Coverage
  Containers
passed #633272321
mingw32-fedora-rawhide-container

00:01:44

passed #633272322
mingw64-fedora-rawhide-container

00:01:47

passed #633272305
x86-centos-7-container

00:01:25

passed #633272308
x86-centos-8-container

00:01:32

passed #633272309
x86-centos-stream-container

00:01:29

passed #633272311
x86-debian-10-container

00:01:27

passed #633272310
x86-debian-9-container

00:01:23

passed #633272312
x86-debian-sid-container

00:01:34

passed #633272313
x86-fedora-31-container

00:01:27

passed #633272314
x86-fedora-32-container

00:01:32

passed #633272316
x86-fedora-rawhide-container

00:01:32

passed #633272317
x86-opensuse-151-container

00:21:05

passed #633272318
x86-ubuntu-1804-container

00:05:00

passed #633272320
x86-ubuntu-2004-container

00:04:34

 
  Builds
passed #633272350
mingw32-fedora-rawhide-git-build

00:12:01

passed #633272351
mingw64-fedora-rawhide-git-build

00:13:33

passed #633272326
x86-centos-7-dist-build

00:01:22

passed #633272328
x86-centos-8-git-build

00:11:13

passed #633272330
x86-centos-stream-dist-build

00:12:39

passed #633272334
x86-debian-10-dist-build

00:01:02

passed #633272332
x86-debian-9-dist-build

00:01:00

passed #633272335
x86-debian-sid-dist-build

00:01:01

passed #633272338
x86-fedora-31-dist-build

00:01:35

passed #633272339
x86-fedora-32-dist-build

00:01:36

passed #633272344
x86-fedora-rawhide-dist-build

00:01:46

passed #633272346
x86-opensuse-151-dist-build

00:01:00

passed #633272347
x86-ubuntu-1804-dist-build

00:00:56

passed #633272348
x86-ubuntu-2004-dist-build

00:00:53