cleanup: spatial indexes in MyISAM

In spatial indexes there can be only one keyseg.
Make it explicit in the code, don't pretend that this can work
with the arbitrary number of keysegs.

storage/myisam/mi_create.c

@@ -273,7 +273,7 @@ int mi_create(const char *name,uint keys,MI_KEYDEF *keydefs,
            so we only need to decrease keydef->keysegs.
            (see recreate_table() in mi_check.c)
         */
-        keydef->keysegs-=sp_segs-1;
+        keydef->keysegs= 1;
       }
 
       for (j=0, keyseg=keydef->seg ; (int) j < keydef->keysegs ;
@@ -287,7 +287,8 @@ int mi_create(const char *name,uint keys,MI_KEYDEF *keydefs,
           goto err_no_lock;
         }
       }
-      keydef->keysegs+=sp_segs;
+      DBUG_ASSERT(keydef->keysegs == 1);
+      keydef->keysegs= sp_segs + 1;
       key_length+=SPLEN*sp_segs;
       length++;                              /* At least one length byte */
       min_key_length_skip+=SPLEN*2*SPDIMS;

storage/myisam/mi_open.c

@@ -377,8 +377,9 @@ MI_INFO *mi_open(const char *name, int mode, uint open_flags)
 	{
 #ifdef HAVE_SPATIAL
 	  uint sp_segs=SPDIMS*2;
-	  share->keyinfo[i].seg=pos-sp_segs;
-	  share->keyinfo[i].keysegs--;
+          share->keyinfo[i].seg= pos - sp_segs;
+          DBUG_ASSERT(share->keyinfo[i].keysegs == sp_segs + 1);
+          share->keyinfo[i].keysegs= sp_segs;
 #else
 	  my_errno=HA_ERR_UNSUPPORTED;
 	  goto err;