BUG#727667 Wrong result with OR + NOT NULL in maria-5.3

- Address review feedback: introduce NO_REF_PART symbolic name, better comments

sql/sql_select.cc

@@ -6564,12 +6564,12 @@ static bool create_ref_for_key(JOIN *join, JOIN_TAB *j,
   j->ref.has_record= FALSE;
   j->ref.null_rejecting= 0;
   j->ref.disable_cache= FALSE;
-  j->ref.null_ref_part= (uint)-1;
+  j->ref.null_ref_part= NO_REF_PART;
   keyuse=org_keyuse;
 
   store_key **ref_key= j->ref.key_copy;
   uchar *key_buff=j->ref.key_buff, *null_ref_key= 0;
-  uint null_ref_part= (uint)-1;
+  uint null_ref_part= NO_REF_PART;
   bool keyuse_uses_no_tables= TRUE;
   if (ftkey)
   {

sql/sql_select.h

@@ -79,6 +79,8 @@ typedef struct keyuse_t {
 
 class store_key;
 
+const int NO_REF_PART= uint(-1);
+
 typedef struct st_table_ref
 {
   bool		key_err;
@@ -113,8 +115,9 @@ typedef struct st_table_ref
   /* null byte position in the key_buf. Used for REF_OR_NULL optimization */
   uchar          *null_ref_key;
   /* 
-    ref_or_null optimization: number of key part can be NULL (there's only one)
-    uint(-1) if no part can be NULL.
+    ref_or_null optimization: number of key part that alternates between
+    the lookup value or NULL (there's only one such part). 
+    If we're not using ref_or_null, the value is NO_REF_PART
   */
   uint           null_ref_part;