Avoid warnings in String::copy when copying string on itself (ok to do)

sql/sql_string.cc

@@ -183,7 +183,16 @@ bool String::copy(const char *str,uint32 arg_length, CHARSET_INFO *cs)
 {
   if (alloc(arg_length))
     return TRUE;
-  if ((str_length=arg_length))
+  if (Ptr == str && arg_length == str_length)
+  {
+    /*
+      This can happen in some cases. This code is here mainly to avoid
+      warnings from valgrind, but can also be an indication of error.
+    */
+    DBUG_PRINT("warning", ("Copying string on itself: %p  %lu",
+                           str, arg_length));
+  }
+  else if ((str_length=arg_length))
     memcpy(Ptr,str,arg_length);
   Ptr[arg_length]=0;
   str_charset=cs;