Fix warnings in platformio (espressif#2960)

Fix : warning: converting to non-pointer type 'int' from NULL [-Wconversion-null] Fix : warning: unused variable 'res' [-Wunused-variable]

Author: luc-github

libraries/FFat/src/FFat.cpp

@@ -68,7 +68,7 @@ void F_Fat::end()
  log_e("Unmounting FFat partition failed! Error: %d", err);
  return;
  }
- _wl_handle = NULL;
+ _wl_handle = 0;
  _impl->mountpoint(NULL);
  }
 }
@@ -109,7 +109,9 @@ size_t F_Fat::totalBytes()
 
  BYTE pdrv = ff_diskio_get_pdrv_wl(_wl_handle);
  char drv[3] = {(char)(48+pdrv), ':', 0};
- FRESULT res = f_getfree(drv, &free_clust, &fs);
+ if ( f_getfree(drv, &free_clust, &fs) != FR_OK){
+ return 0;
+ }
  tot_sect = (fs->n_fatent - 2) * fs->csize;
  sect_size = CONFIG_WL_SECTOR_SIZE;
  return tot_sect * sect_size;
@@ -123,7 +125,9 @@ size_t F_Fat::freeBytes()
 
  BYTE pdrv = ff_diskio_get_pdrv_wl(_wl_handle);
  char drv[3] = {(char)(48+pdrv), ':', 0};
- FRESULT res = f_getfree(drv, &free_clust, &fs);
+ if ( f_getfree(drv, &free_clust, &fs) != FR_OK){
+ return 0;
+ }
  free_sect = free_clust * fs->csize;
  sect_size = CONFIG_WL_SECTOR_SIZE;
  return free_sect * sect_size;