In yoshoku#3 I describe a crash I have been experiencing with numo-pocketfft.  When running RSpec tests in the mb-sound project, my FFT tests crash frequently, and occasionally return incorrect results.  These errors only happen at the `-O3` optimization level.  It appears this is due to Ruby's GC freeing either `x_val` or `z_val` before the function has finished, and other data being written in its place.

This fix uses the RB_GC_GUARD macro to make sure that gcc preserves the `x_val` and `z_val` `VALUE`s in stack or registers so that the GC knows they are still in use.

## Testing

To test the fix I ran `rspec` in a loop in the `fft` branch of the `mb-sound` project, as well as the reproducing script in yoshoku#3.  Before this change, tests would crash frequently.  After this change, tests ran without issue for many iterations.