Fire postcall when a callback thunk re-enters the VM

to_amx_callback_dispatch called vm_callback_precall and then amx.call
but never vm_callback_postcall, unlike vm_load_binary,
vm_execute_function and vm_destroy, which all pair them.

pawnio_km.h documents postcall as running after a VM function executes,
and its only real use is to bracket state taken in precall. An
extension that increments a counter in precall and decrements it in
postcall went permanently unbalanced the first time a module's thunk
re-entered the VM; one that takes a lock in precall would deadlock on
the next call.

Placed before the error check to match the other three sites.