Invalidate FatCache on creation

FatCache on construction is invalid, of course, so mark it as such.

Author: Earle F. Philhower, III

src/FatLib/FatVolume.h

@@ -98,6 +98,9 @@ union cache_t {
  */
 class FatCache {
  public:
+
+  FatCache() { invalidate(); }
+
   /** Cached block is dirty */
   static const uint8_t CACHE_STATUS_DIRTY = 1;
   /** Cashed block is FAT entry and must be mirrored in second FAT. */