Bug#44337 Select query using index merge fails with MCH3601

The storage engine was not correctly handling the case in 
which rnd_pos is executed for a handler without a preceding
rnd_next or index read operation. As a result, an unitialized
file handle was sometimes being passed to the QMY_READ API.

The fix clears the rrnAssocHandle at the beginning of each
read operation and then checks to see whether it has been
set to a valid handle value before attempting to use it
in rnd_pos. If rrnAssocHandle has not been set by a previous
read operation, rnd_pos instead falls back to the use of the
currently active handle.

storage/ibmdb2i/ha_ibmdb2i.cc:
  Bug#44337 Select query using index merge fails with MCH3601
  
  - clear the rrnAssocHandle at the beginning of each
    read operation 
  - checks to see whether it has been set to a valid
    handle value before attempting to use it in rnd_pos

storage/ibmdb2i/ha_ibmdb2i.cc

@@ -898,6 +898,8 @@ int ha_ibmdb2i::index_init(uint idx, bool sorted)
       releaseIndexFile(idx);
   }
   
+  rrnAssocHandle= 0;
+
   DBUG_RETURN(rc); 
 }
 
@@ -1154,6 +1156,8 @@ int ha_ibmdb2i::rnd_init(bool scan)
       releaseDataFile();
   }
   
+  rrnAssocHandle= 0;
+
   DBUG_RETURN(0); // MySQL sometimes does not check the return code, causing 
                   // an assert in ha_rnd_end later on if we return a non-zero
                   // value here. 
@@ -1251,7 +1255,8 @@ int ha_ibmdb2i::rnd_pos(uchar * buf, uchar *pos)
   
   int rc = 0;
 
-  if (activeHandle != rrnAssocHandle) 
+  if (rrnAssocHandle &&
+      (activeHandle != rrnAssocHandle))
   {
     if (activeHandle) releaseActiveHandle();
     rc = useFileByHandle(QMY_UPDATABLE, rrnAssocHandle);