Only delete the cache files if cache is null
Only delete the cache files if cache is null

No need to call getImageCacheService() to get the instance to delete
image cache files if the cache instance is null.
It will renew an instance for cache and lead to the memory leak
if renew too many instances by calling getImageCacheService()
multi-times.

Change-Id: I3ff83f49e9e8a8bc211dc26c773f3ccefc974881
Signed-off-by: Roger Chen <978f296cd7dd9dd5e3d0b6fa935716127e021333@gmail.com>

bmaupin/android-sms-plus