gstinfo: Fix leak in generate_backtrace_trace

Spotted by Laurent Pinchart.
25 jobs for !1412 with fix-generate_backtrace_trace-leak in 28 seconds (queued for 1 second)
latest detached
Status Name Job ID Coverage
  Trigger
manual trigger #16335494
manual
 
  Build Docker
created fedora amd64 docker #16335495
passed gst-indent amd64 docker #16335496

00:00:14

created windows amd64 docker #16335497
windows shell 1809
 
  Preparation
passed gst indent #16335498

00:00:14

 
  Build
created build clang fedora x86_64 #16335500
created build documentation #16335506
created build msys2 #16335503
docker windows 1809 allowed to fail manual
created build nodebug fedora x86_64 #16335499
created build vs2017 amd64 #16335501
docker windows 1809
created build vs2017 x86 #16335502
docker windows 1809
created cerbero trigger #16335507
created gstreamer-full-minimal static build #16335505
created gstreamer-full static build #16335504
 
  Test
created check fedora #16335508
gstreamer
created integration testsuites fedora 1/4 #16335509
gstreamer
created integration testsuites fedora 2/4 #16335510
gstreamer
created integration testsuites fedora 3/4 #16335511
gstreamer
created integration testsuites fedora 4/4 #16335512
gstreamer
created valgrind bad #16335517
gstreamer
created valgrind base #16335514
gstreamer
created valgrind core #16335513
gstreamer
created valgrind ges #16335518
gstreamer
created valgrind good #16335515
gstreamer
created valgrind ugly #16335516
gstreamer