Fix bad checking of values in setStartupBoost()

[Nomagno]

This issue was caused by an oversight in the preconditions of f90c452.
The consequence was that, in online races, if a player didn't press anything during the start sequence, the boost_level value of 1, indicating a normal start with no penalties and no boosts, would get sent from the server to the clients. This would then be processed by Kart::setStartupBoost(uint8_t boost_level), which in fact had a wrong assert that treated a boost level of 1 as a valid value, when it instead resulted on an out-of-bounds memory access to the startup boost vectors when fed into the index formula, crashing every single client.
Fixed by adding an alternate scenario for when the boost level is < 2.

Agreement

By creating a pull request in stk-code, you hereby agree to dual-license your contribution as
GNU General Public License version 3 or any later version and
Mozilla Public License version 2 or any later version.

This includes your previous contribution(s) under the same name of contributor.

Keep the above statement in the pull request comment for agreement.

[Alayan-stk-2]

Thank you for the PR, I didn't see it earlier. The code seems straightforward enough, I'll check it when I have a bit of time for testing.