net: sockets: Simplify if logic in sendto(), fix warning

send()/sendto() aren't "front facing" functions, so when user calls
them, context type hopefully should be already validated by other
functions. They are also on critical path of app/network performance,
so getting rid of extra check helps a little bit too. This also
fixes a warning of "err" possibly being used non-initialized.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>