Remove recursion in UnicodeNormalize.nfc_one, fixing bug 21159.

Author: duerst

lib/unicode_normalize/normalize.rb

@@ -111,17 +111,22 @@ def self.nfc_one(string)
     start = nfd_string[0]
     last_class = CLASS_TABLE[start]-1
     accents = ''
+    result = ''
     nfd_string[1..-1].each_char do |accent|
       accent_class = CLASS_TABLE[accent]
       if last_class<accent_class and composite = COMPOSITION_TABLE[start+accent]
         start = composite
+      elsif accent_class == 0
+        result += start+accents
+        start = accent
+        accents = ''
+        last_class = -1
       else
         accents << accent
         last_class = accent_class
       end
     end
-    accents = nfc_one(accents) if accents.length>1 # TODO: change from recursion to loop
-    hangul_comp_one(start+accents)
+    hangul_comp_one(result+start+accents)
   end
 
   def self.normalize(string, form = :nfc)