net/tftp: Fix dangling memory pointer

net/tftp: Fix dangling memory pointer

The static code analysis tool, Parfait, reported that the valid of
file->data was left referencing memory that was freed by the call to
grub_free(data) where data was initialized from file->data.

To ensure that there is no unintentional access to this memory
referenced by file->data we should set the pointer to NULL.

Signed-off-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>