Better fix for ctype_ldml test failures

Summary:
In D52023, I misunderstood what alloced meant. That fix is causing
problems with other tests. Replace that fix with this one which
works better.

Squash with D52023.

Test Plan:
```
./mtr --mem main.ctype_ldml innodb.innodb_ctype_ldml \
      main.func_str_no_ps main.func_str_debug \
      main.ctype_ucs main.ctype_ucs2_def
./mtr --mem --valgrind main.ctype_ldml innodb.innodb_ctype_ldml \
      main.func_str_no_ps main.func_str_debug \
      main.ctype_ucs main.ctype_ucs2_def
```

Reviewers: jeremycole, weixiang.zhai, inaam-rana, jtolmer, pivanof

Reviewed By: pivanof

Subscribers: santoshb, webscalesql-eng

Differential Revision: https://reviews.facebook.net/D52173

sql/item_sum.cc

@@ -3459,21 +3459,28 @@ Item_func_group_concat::fix_fields(THD *thd, Item **ref)
   if (separator->needs_conversion(separator->length(), separator->charset(),
                                   collation.collation, &offset))
   {
-    uint32 buflen= collation.collation->mbmaxlen * separator->length();
-    uint errors, conv_length;
-    char *buf;
-    String *new_separator;
-
-    if (!(buf= (char*) thd->stmt_arena->alloc(buflen)) ||
-        !(new_separator= new(thd->stmt_arena->mem_root)
-                           String(buf, buflen, collation.collation)))
-      return TRUE;
-
-    conv_length= copy_and_convert(buf, buflen, collation.collation,
-                                  separator->ptr(), separator->length(),
-                                  separator->charset(), &errors);
-    new_separator->length(conv_length);
-    separator= new_separator;
+    if (separator->length())
+    {
+      uint32 buflen= collation.collation->mbmaxlen * separator->length();
+      uint errors, conv_length;
+      char *buf;
+      String *new_separator;
+
+      if (!(buf= (char*) thd->stmt_arena->alloc(buflen)) ||
+          !(new_separator= new(thd->stmt_arena->mem_root)
+                             String(buf, buflen, collation.collation)))
+        return TRUE;
+
+      conv_length= copy_and_convert(buf, buflen, collation.collation,
+                                    separator->ptr(), separator->length(),
+                                    separator->charset(), &errors);
+      new_separator->length(conv_length);
+      separator= new_separator;
+    }
+    else
+    {
+      separator->set_charset(collation.collation);
+    }
   }
 
   if (check_sum_func(thd, ref))

sql/sql_string.h

@@ -527,8 +527,7 @@ class String
 
   inline bool uses_buffer_owned_by(const String *s) const
   {
-    return (alloced && s->alloced &&
-            Ptr >= s->Ptr && Ptr < s->Ptr + s->str_length);
+    return (s->alloced && Ptr >= s->Ptr && Ptr < s->Ptr + s->str_length);
   }
   bool is_ascii() const
   {