ctype_utf8.result, ctype_utf8.test, item.cc:

  Bug#12371 executing prepared statement fails (illegal mix of collations)
  After review fixes.

mysql-test/r/ctype_utf8.result

@@ -1001,6 +1001,9 @@ set @a:='bar';
 execute my_stmt using @a;
 a	b
 bar	kostja
+set @a:=NULL;
+execute my_stmt using @a;
+a	b
 drop table t1;
 CREATE TABLE t1 (
 a varchar(255) NOT NULL default '',

mysql-test/t/ctype_utf8.test

@@ -840,6 +840,8 @@ insert into t1 values ('bar','kostja');
 prepare my_stmt from "select * from t1 where a=?";
 set @a:='bar';
 execute my_stmt using @a;
+set @a:=NULL;
+execute my_stmt using @a;
 drop table t1;
 
 

sql/item.cc

@@ -290,7 +290,13 @@ Item *Item_param::safe_charset_converter(CHARSET_INFO *tocs)
   {
     Item_string *conv;
     uint conv_errors;
-    String tmp, cstr, *ostr= val_str(&tmp);
+    char buf[MAX_FIELD_WIDTH];
+    String tmp(buf, sizeof(buf), &my_charset_bin);
+    String cstr, *ostr= val_str(&tmp);
+    /*
+      As safe_charset_converter is not executed for
+      a parameter bound to NULL, ostr should never be 0.
+    */
     cstr.copy(ostr->ptr(), ostr->length(), ostr->charset(), tocs, &conv_errors);
     if (conv_errors || !(conv= new Item_string(cstr.ptr(), cstr.length(),
                                                cstr.charset(),