This seems not possible. evacuate_fast32 is only called when h.growing() is true, which is when h.oldbuckets != nil. But evacuate_fast32 passes h.oldbuckets to evacuated, and evacuated then nil-ptr panics on its arg.
So I guess it is possible that there's a compiler bug. I'd have to grovel through some assembly.
A race could cause this, but the code in question is operating on a goroutine-local map.
Could also be a machine flake, hard to be sure. Have we seen this failure mode anywhere else?