Fixing crashing bugs as reported in bugs database

#195

mysql-test/r/cast.result

@@ -40,3 +40,6 @@ cast("1:2:3" as TIME) = "1:02:03"
 select cast(NULL as DATE);
 cast(NULL as DATE)
 NULL
+select cast(NULL as BINARY);
+cast(NULL as BINARY)
+NULL

mysql-test/t/cast.test

@@ -21,3 +21,4 @@ select cast("2001-1-1" as date) = "2001-01-01";
 select cast("2001-1-1" as datetime) = "2001-01-01 00:00:00";
 select cast("1:2:3" as TIME) = "1:02:03";
 select cast(NULL as DATE);
+select cast(NULL as BINARY);

sql/item_cmpfunc.cc

@@ -769,7 +769,8 @@ Item_func_if::val_str(String *str)
 {
   Item *arg= args[0]->val_int() ? args[1] : args[2];
   String *res=arg->val_str(str);
-  res->set_charset(charset());
+  if (res)
+    res->set_charset(charset());
   null_value=arg->null_value;
   return res;
 }

sql/item_strfunc.h

@@ -512,7 +512,8 @@ public:
   {
     String *tmp=args[0]->val_str(a);
     null_value=args[0]->null_value;
-    tmp->set_charset(&my_charset_bin);
+    if (tmp)
+      tmp->set_charset(&my_charset_bin);
     return tmp;
   }
   void fix_length_and_dec()