There's no need to initialize extp to NULL, it gets set in either path,
and this code should be extra small/fast.

Hopefully, this will shut Coverity up as well, it's saying:

Explicit null dereferenced (FORWARD_NULL)
assign_zero: Assigning: "*extp" = "NULL".
var_deref_op: Dereferencing null pointer "*extp".
enum don't need forward declarations.