mruby-compar-ext/compar.rb (clamp): fix range support.

Also, raise error if any comparison among min, max, and self
returns nil.

mrbgems/mruby-compar-ext/mrblib/compar.rb

@@ -43,13 +43,13 @@ module Comparable
   def clamp(min, max=nil)
     if max.nil?
       if min.kind_of?(Range)
-        max = min.begin
+        max = min.end
         if max.nil?
           max = self
         elsif min.exclude_end?
           raise ArgumentError, "cannot clamp with an exclusive range"
         end
-        min = min.end
+        min = min.begin
         if min.nil?
           min = self
         end
@@ -57,17 +57,24 @@ module Comparable
         raise TypeError, "wrong argument type #{min.class}"
       end
     end
-    if (min <=> max) > 0
+    c = min <=> max
+    if c.nil?
+      raise ArgumentError, "comparison of #{min.class} with #{max.class} failed"
+    elsif c > 0
       raise ArgumentError, "min argument must be smaller than max argument"
     end
     c = self <=> min
-    if c == 0
+    if c.nil?
+      raise ArgumentError, "comparison of #{self.class} with #{min.class} failed"
+    elsif c == 0
       return self
     elsif c < 0
       return min
     end
     c = self <=> max
-    if c > 0
+    if c.nil?
+      raise ArgumentError, "comparison of #{self.class} with #{max.class} failed"
+    elsif c > 0
       return max
     else
       return self