std.crypto siphash: fix assertion on the size of output buffer
std.crypto siphash: fix assertion on the size of output buffer

the logic was backwards

zig-lang/zig,andrewrk/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,raulgrell/zig,raulgrell/zig,zig-lang/zig,raulgrell/zig,andrewrk/zig