std.io: fix compile error when InStream has empty error set
std.io: fix compile error when InStream has empty error set

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