Wrong if condition in schemas.go

[lukedirtwalker]

The following if condition
https://github.com/capnproto/go-capnproto2/blob/4a2d721f0c4b15f3e369dc4a82d604ef605d7da5/schemas/schemas.go#L114-L115
is wrong. The error is assigned to r.err but the check below is on err which was previously checked.
Not sure what the correct solution would be, probably the if should just check for r.err, or is the assignment to r.err wrong in the first place?

Found with go nilness analyzer (https://godoc.org/golang.org/x/tools/go/analysis/passes/nilness)

[zombiezen]

Thanks for the report! You are correct that r.err is what should be checked there, want to send a PR?

[lukedirtwalker]

@zombiezen opened #148