dosfs: use calloc instead of malloc

calloc zeroes out the memory it allocates, which helps avoid
segfaults related to linked list traversal.

src/dosfs.c

@@ -126,7 +126,7 @@ dos_listdir(dosfs_t *fsd, unsigned int offset)
         }
 
         if (i > 0) {
-            cur->next = malloc(sizeof(dosfile_t));
+            cur->next = calloc(1, sizeof(dosfile_t));
             if (cur->next == NULL) {
                 DPRINTF(("inner malloc failed!?\n"));
                 res = -1;