block/vvfat: Fix crash when reporting error about too many files in directory

block/vvfat: Fix crash when reporting error about too many files in directory

When using the vvfat driver with a directory that contains too many files,
QEMU currently crashes. This can be triggered like this for example:

 mkdir /tmp/vvfattest
 cd /tmp/vvfattest
 for ((x=0;x<=513;x++)); do mkdir $x; done
 qemu-system-x86_64 -drive \
   file.driver=vvfat,file.dir=.,read-only=on,media=cdrom

Seems like read_directory() is changing the mapping->path variable. Make
sure we use the right pointer instead.

Signed-off-by: Thomas Huth <[email protected]>
Signed-off-by: Kevin Wolf <[email protected]>