net: 6lo: Fix memory corruption during uncompression

A memory corruption could happen in `uncompress_IPHC_header` function,
when data was moved to make place in the net buffer for the uncompressed
IPv6/UDP header.

The size of data being moved should only contain the original data size,
not incremented by the amount of space needed to expand the header,
which was already added to the net buffer size. In result, the `memmove`
operation could exceed the allocated net buffer and cause memory
corruption.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>