Patch for Bug#50511 (Sometimes wrong handling of user variables containing NULL).

The bug happened under the following condition:
  - there was a user variable of type REAL, containing NULL value
  - there was a table with a NOT_NULL column of any type but REAL, having
    default value (or auto increment);
  - a row was inserted into the table with the user variable as value.
    A warning was emitted here.

The problem was that handling of NULL values of REAL type was not properly
implemented: it didn't expect that REAL NULL value can be assigned to other
data type.

Basically, the problem was that set_field_to_null() was used instead of
set_field_to_null_with_conversions().

The fix is to use the right function, or more generally, to allow conversion of
REAL NULL values to other data types.

mysql-test/r/null.result

@@ -94,7 +94,7 @@ Warnings:
 Warning	1265	Data truncated for column 'd' at row 1
 UPDATE t1 SET d=1/NULL;
 Warnings:
-Warning	1265	Data truncated for column 'd' at row 1
+Warning	1048	Column 'd' cannot be null
 UPDATE t1 SET d=NULL;
 Warnings:
 Warning	1048	Column 'd' cannot be null

mysql-test/r/user_var.result

@@ -429,3 +429,34 @@ INSERT INTO t1 VALUES (1);
 INSERT INTO t1 VALUES (1);
 DROP TABLE t1;
 End of 5.1 tests
+DROP TABLE IF EXISTS t1;
+CREATE TABLE t1(f1 INT AUTO_INCREMENT, PRIMARY KEY(f1));
+INSERT INTO t1 SET f1 = NULL ;
+SET @aux = NULL ;
+INSERT INTO t1 SET f1 = @aux ;
+SET @aux1 = 0.123E-1;
+SET @aux1 = NULL;
+INSERT INTO t1 SET f1 = @aux1 ;
+SELECT * FROM t1;
+f1
+1
+2
+3
+DROP TABLE t1;
+CREATE TABLE t1(f1 VARCHAR(257) , f2 INT, PRIMARY KEY(f2));
+CREATE TRIGGER trg1 BEFORE INSERT ON t1 FOR EACH ROW SET @aux = 1;
+SET @aux = 1;
+SET @aux = NULL;
+INSERT INTO test.t1 (f1, f2) VALUES (1, 1), (@aux, 2);
+SET @aux = 'text';
+SET @aux = NULL;
+INSERT INTO t1(f1, f2) VALUES (1, 3), (@aux, 4);
+SELECT f1, f2 FROM t1 ORDER BY f2;
+f1	f2
+1	1
+1	2
+1	3
+1	4
+DROP TRIGGER trg1;
+DROP TABLE t1;
+End of 5.5 tests

mysql-test/t/user_var.test

@@ -327,3 +327,44 @@ INSERT INTO t1 VALUES (1);
 DROP TABLE t1;
 
 --echo End of 5.1 tests
+
+#
+# Bug#50511: Sometimes wrong handling of user variables containing NULL.
+#
+
+--disable_warnings
+DROP TABLE IF EXISTS t1;
+--enable_warnings
+
+CREATE TABLE t1(f1 INT AUTO_INCREMENT, PRIMARY KEY(f1));
+
+INSERT INTO t1 SET f1 = NULL ;
+
+SET @aux = NULL ;
+INSERT INTO t1 SET f1 = @aux ;
+
+SET @aux1 = 0.123E-1;
+SET @aux1 = NULL;
+INSERT INTO t1 SET f1 = @aux1 ;
+
+SELECT * FROM t1;
+
+DROP TABLE t1;
+
+CREATE TABLE t1(f1 VARCHAR(257) , f2 INT, PRIMARY KEY(f2));
+CREATE TRIGGER trg1 BEFORE INSERT ON t1 FOR EACH ROW SET @aux = 1;
+
+SET @aux = 1; # INT
+SET @aux = NULL;
+INSERT INTO test.t1 (f1, f2) VALUES (1, 1), (@aux, 2);
+
+SET @aux = 'text'; # STRING
+SET @aux = NULL;
+INSERT INTO t1(f1, f2) VALUES (1, 3), (@aux, 4);
+
+SELECT f1, f2 FROM t1 ORDER BY f2;
+
+DROP TRIGGER trg1;
+DROP TABLE t1;
+
+--echo End of 5.5 tests

sql/item.cc

@@ -5423,7 +5423,7 @@ int Item::save_in_field(Field *field, bool no_conversions)
   {
     double nr= val_real();
     if (null_value)
-      return set_field_to_null(field);
+      return set_field_to_null_with_conversions(field, no_conversions);
     field->set_notnull();
     error=field->store(nr);
   }