Fix bug 3006812: ensure exif cache is initialized even when all files are unreadable.

Fix bug 3006812: ensure exif cache is initialized even when all files are unreadable.

Add a new function exif_init_cache() and call it at top of exif_read_fd(),
the issue was caused by a return NULL on unreadable file, before initializinf exif_cache
and when exif_free_fd() is called, file_cache_put() was called on invalid cache.