Fix real keyread count for partitions

Sergei's commit ac6b3c44 implemented handler status counters
compensation for underlying handlers like ha_partition.
`index_read_idx_map` is missing there, but it should have been fixed as
well (proof: ha_partition::index_read_idx_map never calls
ha_partition::index_read_map).

Note: all this compensation logic could be broken for subpartitions! (We
can experience double decrement)

mysql-test/main/partition_explicit_prune.result

@@ -546,7 +546,7 @@ SELECT * FROM INFORMATION_SCHEMA.SESSION_STATUS
 WHERE VARIABLE_NAME LIKE 'HANDLER_%' AND VARIABLE_VALUE > 0;
 VARIABLE_NAME	VARIABLE_VALUE
 HANDLER_COMMIT	1
-HANDLER_READ_KEY	2
+HANDLER_READ_KEY	1
 HANDLER_TMP_WRITE	24
 HANDLER_UPDATE	2
 HANDLER_WRITE	2
@@ -628,7 +628,7 @@ WHERE VARIABLE_NAME LIKE 'HANDLER_%' AND VARIABLE_VALUE > 0;
 VARIABLE_NAME	VARIABLE_VALUE
 HANDLER_COMMIT	4
 HANDLER_DELETE	2
-HANDLER_READ_KEY	3
+HANDLER_READ_KEY	2
 HANDLER_READ_NEXT	1
 HANDLER_READ_RND_NEXT	112
 HANDLER_ROLLBACK	1
@@ -651,7 +651,7 @@ VARIABLE_NAME	VARIABLE_VALUE
 HANDLER_COMMIT	5
 HANDLER_DELETE	2
 HANDLER_READ_FIRST	1
-HANDLER_READ_KEY	3
+HANDLER_READ_KEY	2
 HANDLER_READ_NEXT	4
 HANDLER_READ_RND_NEXT	140
 HANDLER_ROLLBACK	1
@@ -671,7 +671,7 @@ VARIABLE_NAME	VARIABLE_VALUE
 HANDLER_COMMIT	5
 HANDLER_DELETE	2
 HANDLER_READ_FIRST	1
-HANDLER_READ_KEY	3
+HANDLER_READ_KEY	2
 HANDLER_READ_NEXT	4
 HANDLER_READ_RND_NEXT	168
 HANDLER_ROLLBACK	1

sql/ha_partition.cc

@@ -5837,6 +5837,7 @@ int ha_partition::index_read_idx_map(uchar *buf, uint index,
 {
   int error= HA_ERR_KEY_NOT_FOUND;
   DBUG_ENTER("ha_partition::index_read_idx_map");
+  decrement_statistics(&SSV::ha_read_key_count);
 
   if (find_flag == HA_READ_KEY_EXACT)
   {