Bug#49134 5.1 server segfaults with 2byte collation file

Problem: add_collation did not check that cs->number is smaller
than the number of elements in the array all_charsets[],
so server could crash when loading an Index.xml file with
a collation ID greater the number of elements 
(for example when downgrading from 5.5).

Fix: adding a condition to check that cs->number is not out of valid range.

mysql-test/std_data/Index.xml

@@ -8,6 +8,13 @@
       </rules>
     </collation>
 
+   <collation name="utf8_hugeid_ci" id="2047000000">
+      <rules>
+        <reset>a</reset>
+        <s>b</s>
+      </rules>
+   </collation>
+
   </charset>
 
   <charset name="ucs2">

mysys/charset.c

@@ -220,7 +220,8 @@ copy_uca_collation(CHARSET_INFO *to, CHARSET_INFO *from)
 static int add_collation(CHARSET_INFO *cs)
 {
   if (cs->name && (cs->number ||
-                   (cs->number=get_collation_number_internal(cs->name))))
+                   (cs->number=get_collation_number_internal(cs->name))) &&
+      cs->number < array_elements(all_charsets))
   {
     if (!all_charsets[cs->number])
     {