Bluetooth: Mesh: Fix uninitialized field when cancelling transfer blob

Initialize the missing chunk field when canceling a BLOB Transfer.
If you canceled a BLOB Transfer, the missing_chunk was not cleared in
the next BLOB Transfer. This can make it look like there are still
missing chunks, even though all chunks in the block have already been
received, and it does cause bugs.

Signed-off-by: Junho Lee <tot0roprog@gmail.com>