fftools/cmdutils: use strcpy directly, the length is computed already

There is no need to scan for NULL, if we inject it ourselves.

Fixes: warning: 'strncat' specified bound 10 equals source length [-Wstringop-overflow=]
Signed-off-by: Kacper Michajłow <kasper93@gmail.com>

fftools/cmdutils.c

@@ -962,8 +962,7 @@ FILE *get_preset_file(char *filename, size_t filename_size,
                     datadir, desired_size, sizeof *datadir);
                 if (new_datadir) {
                     datadir = new_datadir;
-                    datadir[datadir_len] = 0;
-                    strncat(datadir, "/ffpresets",  desired_size - 1 - datadir_len);
+                    strcpy(datadir + datadir_len, "/ffpresets");
                     base[2] = datadir;
                 }
             }