Two small const correctness changes

Hey:

These changes make ZVAL_COPY and ZVAL_COPY_VALUE work without warning
for places where the zval that is copied is const:
https://gist.github.com/morrisonlevi/69156fdf93e6bc82f9a0

In the default build there do not seem to be any places where this
causes a warning, but I noticed it in an extension I'm writing. I do
not yet have Zend Engine karma; hence posting to the list.
done.

thanks

Thank you for committing this.