cmd/nvedit.c: Update input handling to cover overflow cases

When we have multiple messages provided, we need to be sure that we do
not exceed the length of our 'message' buffer.  In the for loop, make
sure that pos is not larger than message.  Only copy in at most however
much of the message buffer remains.  Finally, if we have not reached the
end of the message buffer, put in a space and NULL, and if we have,
ensure the buffer is now NULL termined.

Reported-by: Coverity (CID: 165116)
Signed-off-by: Tom Rini <trini@konsulko.com>