Build: Added a missing const.

Building --with-debug should now work again.
Compiler: Fixed NULL-deref in tree optimizer.

CAR for a F_*_EQ node may be NULL if the variable hasn't been declared.

Fixes [bug 7397].
Fixed decrypt_char
Move strdup from pike_memory to port.
pike_memory_locks isn't used. Remove that and the now always unused initializer.
Simplified
Fixed the fix.