A DecodeCustomTypeFunc returning nil to mean "leave this pointer
field unset" panicked in reflect.Value.Set, since reflect.ValueOf(nil)
is the invalid zero Value. Guard against a nil return before Set.
This is the panic from #68/#69: the earlier fix routed pointer fields
through the generic Ptr recursion instead of guarding nil, which
discarded a custom function's own populated pointer result and had
to be reverted. Guarding val == nil directly fixes the panic without
touching the case that broke.
upstream
dualfroz