driver(container): fix conditional statement for error handling

[crazy-max]

fixes #2177

When creating a builder and setting memory limit, it panics:

$ docker --debug buildx create --name foo --driver-opt "memory=3g"
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x48 pc=0x120c656]

goroutine 46 [running]:
github.com/docker/buildx/builder.(*Node).loadData(0xc000429d40, {0x266fc00, 0xc00025ebd0})
        github.com/docker/buildx/builder/node.go:254 +0x56
github.com/docker/buildx/builder.(*Builder).LoadNodes.(*Builder).LoadNodes.func2.func3()
        github.com/docker/buildx/builder/node.go:154 +0x625
golang.org/x/sync/errgroup.(*Group).Go.func1()
        golang.org/x/[email protected]/errgroup/errgroup.go:75 +0x56
created by golang.org/x/sync/errgroup.(*Group).Go in goroutine 1
        golang.org/x/[email protected]/errgroup/errgroup.go:72 +0x96

Conditional statement for error handling was not set correctly.

[tonistiigi]

This is bit weird construction as it always sets the value and then error in handled later. But coming from cli so nothing we can do about it.