Fix for a NAN problem in FORMAT(...) string function ..

mysql-test/r/func_misc.result

@@ -10,3 +10,6 @@ inet_aton("255.255.255.255.255")	inet_aton("255.255.1.255")	inet_aton("0.1.255")
 select inet_ntoa(1099511627775),inet_ntoa(4294902271),inet_ntoa(511);
 inet_ntoa(1099511627775)	inet_ntoa(4294902271)	inet_ntoa(511)
 NULL	255.255.1.255	0.0.1.255
+select format("nan",2);
+format("nan",2)
+nan

mysql-test/t/func_misc.test

@@ -7,3 +7,4 @@ select format(1.5555,0),format(123.5555,1),format(1234.5555,2),format(12345.5555
 select inet_ntoa(inet_aton("255.255.255.255.255.255.255.255"));
 select inet_aton("255.255.255.255.255"),inet_aton("255.255.1.255"),inet_aton("0.1.255");
 select inet_ntoa(1099511627775),inet_ntoa(4294902271),inet_ntoa(511);
+select format("nan",2);

sql/item_strfunc.cc

@@ -1477,7 +1477,7 @@ String *Item_func_format::val_str(String *str)
   if (nr < 0)
     str_length--;				// Don't count sign
   length=str->length()+(diff=(str_length- dec-1)/3);
-  if (diff)
+  if (diff && diff < 330) // size of buff ...
   {
     char *tmp,*pos;
     str=copy_if_not_alloced(&tmp_str,str,length);