Fix handling of NULL fields in FIELD(). (Bug #10944)

mysql-test/r/func_str.result:
  Update results
mysql-test/t/func_str.test:
  Add regression tests
sql/item_func.cc:
  Handle NULL as first argument to FIELD()

mysql-test/r/func_str.result

@@ -783,3 +783,9 @@ id	aes_decrypt(str, 'bar')
 1	foo
 2	NULL
 DROP TABLE t1, t2;
+select field(0,NULL,1,0), field("",NULL,"bar",""), field(0.0,NULL,1.0,0.0);
+field(0,NULL,1,0)	field("",NULL,"bar","")	field(0.0,NULL,1.0,0.0)
+3	3	3
+select field(NULL,1,2,NULL), field(NULL,1,2,0);
+field(NULL,1,2,NULL)	field(NULL,1,2,0)
+0	0

mysql-test/t/func_str.test

@@ -521,3 +521,9 @@ SELECT t1.id, aes_decrypt(str, 'bar') FROM t1, t2 WHERE t1.id = t2.id
 
 DROP TABLE t1, t2;
 
+
+#
+# Bug #10944: Mishandling of NULL arguments in FIELD()
+#
+select field(0,NULL,1,0), field("",NULL,"bar",""), field(0.0,NULL,1.0,0.0);
+select field(NULL,1,2,NULL), field(NULL,1,2,0);

sql/item_func.cc

@@ -1488,6 +1488,10 @@ void Item_func_locate::print(String *str)
 longlong Item_func_field::val_int()
 {
   DBUG_ASSERT(fixed == 1);
+
+  if (args[0]->null_value)
+    return 0;
+
   if (cmp_type == STRING_RESULT)
   {
     String *field;
@@ -1505,7 +1509,7 @@ longlong Item_func_field::val_int()
     longlong val= args[0]->val_int();
     for (uint i=1; i < arg_count ; i++)
     {
-      if (val == args[i]->val_int())
+      if (!args[i]->null_value && val == args[i]->val_int())
         return (longlong) (i);
     }
   }
@@ -1514,7 +1518,7 @@ longlong Item_func_field::val_int()
     double val= args[0]->val();
     for (uint i=1; i < arg_count ; i++)
     {
-      if (val == args[i]->val())
+      if (!args[i]->null_value && val == args[i]->val())
         return (longlong) (i);
     }
   }