fcformat: fix compiler warnings with clang on Android

fcformat.c:762:44: warning: expression which evaluates to zero treated as a null pointer constant of type 'FcChar8 *' (aka 'unsigned char *') [-Wnon-literal-null-conversion]
                if (!FcNameUnparseValue (buf, &l->value, '\0'))
                                                         ^~~~
fcformat.c:769:38: warning: expression which evaluates to zero treated as a null pointer constant of type 'FcChar8 *' (aka 'unsigned char *') [-Wnon-literal-null-conversion]
            FcNameUnparseValueList (buf, l, '\0');
                                            ^~~~
10 jobs for android-compiler-warnings in 7 minutes and 19 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #6403509
gst-macos-10.15
meson macos

00:03:03

passed #6403506
meson-mingw-w64-build

00:04:45

manual #6403510
docker windows 1809 allowed to fail manual
meson msys2
passed #6403504
meson-shared-build

00:04:47

passed #6403505
meson-static-build

00:05:47

passed #6403507
docker windows 1809
meson vs2017 amd64

00:01:24

passed #6403508
docker windows 1809
meson vs2017 x86

00:01:22

passed #6403503
mingw-build

00:06:20

passed #6403501
shared-build

00:07:19

passed #6403502
static-build

00:05:57