navmenu() trashes $modinfo

The navmenu() function makes a change to the $modinfo variable obtained by get_fast_modinfo. The $cms[id] entry for the current module has its name overwritten to 'Jump to...'.

These changes are intended to be local to the function, however it is actually working on a reference to the cached modinfo which means that anyone who calls get_fast_modinfo later gets the messed-up data!

I don't know if this has any code effect in moodle 1.9 but as it is a bugfix maybe is worth doing. Should definitely go into head.

I have attached the patch - simplest/most efficient fix was simply to save the previous value and restore it afterwards (rather than trying to clone the variable or anything).