ring-buffer-iterator: fix postfix increment/decrement

this silently did the wrong thing on g++ (precedence of postfix ++/--)
and didn't compile on clang++. The correct solution is to
prefix-increment, which has no precedence issues and calls the correct
function. Postfix-increment with correct precedence would have resulted
in a stack overflow anyway.

include/ring-buffer-iterator.h

@@ -61,7 +61,7 @@ public:
     }
     CONSTEXPR ring_buffer_iterator operator++(int) NOEXCEPT {
         ring_buffer_iterator old = *this;
-        *this++;
+        ++*this;
         return old;
     }
     CONSTEXPR ring_buffer_iterator& operator--() NOEXCEPT {
@@ -70,7 +70,7 @@ public:
     }
     CONSTEXPR ring_buffer_iterator operator--(int) NOEXCEPT {
         ring_buffer_iterator old = *this;
-        *this--;
+        --*this;
         return old;
     }
 
@@ -161,7 +161,7 @@ public:
     }
     CONSTEXPR ring_buffer_const_iterator operator++(int) NOEXCEPT {
         ring_buffer_const_iterator old = *this;
-        *this++;
+        ++*this;
         return old;
     }
     CONSTEXPR ring_buffer_const_iterator& operator--() NOEXCEPT {
@@ -170,7 +170,7 @@ public:
     }
     CONSTEXPR ring_buffer_const_iterator operator--(int) NOEXCEPT {
         ring_buffer_const_iterator old = *this;
-        *this--;
+        --*this;
         return old;
     }