clamp: added variant clamp_nonzero() to only clamp non-zero values

This is useful for clamping parameter values to a range,
but where zero is a special case (for example to disable
the parameter).

Signed-off-by: James Yonan <james@openvpn.net>

openvpn/common/clamp.hpp

@@ -36,4 +36,13 @@ namespace openvpn {
       return value;
   }
 
+  // like clamp() above, but only clamp non-zero values
+  template <typename T>
+  T clamp_nonzero(T value, T low, T high)
+  {
+    if (value)
+      return clamp(value, low, high);
+    else
+      return value;
+  }
 }