Fix leaks in fcxml.c, fc-match.c, and tests.

Fix leaks reported by AddressSanitizer when running 'make check'.
10 jobs for fix_leaks in 7 minutes and 21 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #6158050
gst-macos-10.15
meson macos

00:03:09

passed #6158047
meson-mingw-w64-build

00:04:44

manual #6158051
docker windows 1809 allowed to fail manual
meson msys2
passed #6158045
meson-shared-build

00:05:13

passed #6158046
meson-static-build

00:05:38

passed #6158048
docker windows 1809
meson vs2017 amd64

00:01:19

passed #6158049
docker windows 1809
meson vs2017 x86

00:01:20

passed #6158044
mingw-build

00:05:56

passed #6158042
shared-build

00:07:21

passed #6158043
static-build

00:05:44