sntp: avoid use of uninitialized variable

When we use the ntpserverip environment variable argv[1] may not be set.
Printing the error message using the NetNtpServerIP variable ensures the
correct output in both cases.

Signed-off-by: Luuk Paulussen <luuk.paulussen@alliedtelesis.co.nz>
Acked-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Cc: Ben Warren <biggerbadderben@gmail.com>
Acked-by: Detlev Zundel <dzu@denx.de>