RACE: call_with_interpreter() can lose track of the Pike_interpreter_pointer.

Imported from https://youtrack.roxen.com/issue/PIKE-120

Reported by @grubba

call_with_interpreter() clears Pike_interpreter_pointer without holding the interpreter lock.

This may cause other threads to lose their interpreter state.

Note that this is only an issue with pike binaries compiled {{--with-debug}}.