BUG#19618: Crash for unsigned_col NOT IN (-1, ... )

- When manually constructing a SEL_TREE for "t.key NOT IN(...)", take into account that 
  get_mm_parts may return a tree with type SEL_TREE::IMPOSSIBLE
- Added missing OOM checks
- Added comments

mysql-test/r/func_in.result

@@ -326,3 +326,20 @@ deallocate prepare s;
 set @str=NULL;
 drop table t2;
 drop table t1;
+create table t1 (
+some_id smallint(5) unsigned,
+key (some_id)
+);
+insert into t1 values (1),(2);
+select some_id from t1 where some_id not in(2,-1);
+some_id
+1
+select some_id from t1 where some_id not in(-4,-1,-4);
+some_id
+1
+2
+select some_id from t1 where some_id not in(-4,-1,3423534,2342342);
+some_id
+1
+2
+drop table t1;