When reading with `read_cluster` we get the `mapping` with
`find_mapping_for_cluster` and then we call `open_file` for this
mapping.
The issue appear when its the same file, but a second cluster that is
not immediately after it, imagine clusters `500 -> 503`, this will give
us 2 mappings one has the range `500..501` and another `503..504`, both
point to the same file, but different offsets.

When we don't open the file since the path is the same, we won't assign
`s->current_mapping` and thus accessing way out of bound of the file.

From our example above, after `open_file` (that didn't open anything) we
will get the offset into the file with
`s->cluster_size*(cluster_num-s->current_mapping->begin)`, which will
give us `0x2000 * (504-500)`, which is out of bound for this mapping and
will produce some issues.

Signed-off-by: Amjad Alsharafi <amjadsharafi10@gmail.com>
---
 block/vvfat.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/block/vvfat.c b/block/vvfat.c
index f0642ac3e4..8b4d162aa1 100644
--- a/block/vvfat.c
+++ b/block/vvfat.c
@@ -1360,15 +1360,24 @@ static int open_file(BDRVVVFATState* s,mapping_t* mapping)
 {
     if(!mapping)
         return -1;
+    int new_path = 1;
     if(!s->current_mapping ||
-            strcmp(s->current_mapping->path,mapping->path)) {
-        /* open file */
-        int fd = qemu_open_old(mapping->path,
+            s->current_mapping->first_mapping_index
+                != mapping->first_mapping_index ||
+            (new_path = strcmp(s->current_mapping->path, mapping->path))) {
+
+        if (new_path) {
+            /* open file */
+            int fd = qemu_open_old(mapping->path,
                                O_RDONLY | O_BINARY | O_LARGEFILE);
-        if(fd<0)
-            return -1;
-        vvfat_close_current_file(s);
-        s->current_fd = fd;
+            if (fd < 0) {
+                return -1;
+            }
+            vvfat_close_current_file(s);
+
+            s->current_fd = fd;
+        }
+        assert(s->current_fd);
         s->current_mapping = mapping;
     }
     return 0;
-- 
2.45.1