exactness: make sure we handle a negative file descriptor

In an error case the fd could be negative here and we should check
before feeding it into fdopen(). This is the same patch we used in
recorder.c

CID: 1422194

Reviewed-by: Marcel Hollerbach <mail@marcel-hollerbach.de>
Differential Revision: https://phab.enlightenment.org/D11728

src/bin/exactness/player.c

@@ -983,6 +983,7 @@ _setup_font_settings(const char *fonts_dir)
         if (chosen_fonts)
           {
              int tmp_fd = eina_file_mkstemp("/tmp/fonts_XXXXXX.conf", &fonts_conf_name);
+             if (tmp_fd < 0) return EINA_FALSE;
              FILE *tmp_f = fdopen(tmp_fd, "wb");
              fprintf(tmp_f,
                    "<?xml version=\"1.0\"?>\n<!DOCTYPE fontconfig SYSTEM \"fonts.dtd\">\n<fontconfig>\n"