Bug #26537: item_unhex() was not expected

to return NULL for non-NULL arguments.
This is not the case as it can return NULL
for invalid hexidecimal strings.
Fixed by setting the maybe_null flag.

mysql-test/r/func_str.result

@@ -1940,4 +1940,10 @@ abcxx
 select lpad('abc', cast(5 as unsigned integer), 'x');
 lpad('abc', cast(5 as unsigned integer), 'x')
 xxabc
+SELECT UNHEX('G');
+UNHEX('G')
+NULL
+SELECT UNHEX('G') IS NULL;
+UNHEX('G') IS NULL
+1
 End of 5.0 tests

mysql-test/t/func_str.test

@@ -1008,4 +1008,10 @@ select repeat('a', cast(2 as unsigned int));
 select rpad('abc', cast(5 as unsigned integer), 'x');
 select lpad('abc', cast(5 as unsigned integer), 'x');
 
+#
+# Bug #26537: UNHEX() IS NULL comparison fails
+#
+SELECT UNHEX('G');
+SELECT UNHEX('G') IS NULL;
+
 --echo End of 5.0 tests

sql/item_strfunc.h

@@ -605,7 +605,11 @@ class Item_func_unhex :public Item_str_func
 {
   String tmp_value;
 public:
-  Item_func_unhex(Item *a) :Item_str_func(a) {}
+  Item_func_unhex(Item *a) :Item_str_func(a) 
+  { 
+    /* there can be bad hex strings */
+    maybe_null= 1; 
+  }
   const char *func_name() const { return "unhex"; }
   String *val_str(String *);
   void fix_length_and_dec()