Invalid nicknames containing only spaces will result in an underflow in
a memcpy size calculation, subsequently destroying the heap and
panicking.

v2 also catches the case where the provided nickname is longer than the
buffer size, which can result in controllable heap corruption.
Signed-off-by: NDan Rosenberg <drosenberg@vsecurity.com>
Cc: stable@kernel.org
Signed-off-by: NDavid S. Miller <davem@davemloft.net>