Fix const_reverse_iterator constructor (pass const ptr)

src/prevector.h

@@ -131,7 +131,7 @@ class prevector {
         typedef const T* pointer;
         typedef const T& reference;
         typedef std::bidirectional_iterator_tag iterator_category;
-        const_reverse_iterator(T* ptr_) : ptr(ptr_) {}
+        const_reverse_iterator(const T* ptr_) : ptr(ptr_) {}
         const_reverse_iterator(reverse_iterator x) : ptr(&(*x)) {}
         const T& operator*() const { return *ptr; }
         const T* operator->() const { return ptr; }