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');
^~~~