ITS#8062 fix rebalance

(Probably fixes the ITS, definitely fixes a bug) when collapsing
the root page, fixups of other cursors was incomplete.

libraries/liblmdb/mdb.c

@@ -7717,12 +7717,12 @@ mdb_rebalance(MDB_cursor *mc)
 						m3 = m2;
 					if (m3 == mc || m3->mc_snum < mc->mc_snum) continue;
 					if (m3->mc_pg[0] == mp) {
-						m3->mc_snum--;
-						m3->mc_top--;
 						for (i=0; i<m3->mc_snum; i++) {
 							m3->mc_pg[i] = m3->mc_pg[i+1];
 							m3->mc_ki[i] = m3->mc_ki[i+1];
 						}
+						m3->mc_snum--;
+						m3->mc_top--;
 					}
 				}
 			}
@@ -7792,7 +7792,11 @@ mdb_rebalance(MDB_cursor *mc)
 		} else {
 			oldki += NUMKEYS(mn.mc_pg[mn.mc_top]);
 			mn.mc_ki[mn.mc_top] += mc->mc_ki[mn.mc_top] + 1;
+			/* We want mdb_rebalance to find mn when doing fixups */
+			mn.mc_next = mc->mc_txn->mt_cursors[mc->mc_dbi];
+			mc->mc_txn->mt_cursors[mc->mc_dbi] = &mn;
 			rc = mdb_page_merge(mc, &mn);
+			mc->mc_txn->mt_cursors[mc->mc_dbi] = mn.mc_next;
 			mdb_cursor_copy(&mn, mc);
 		}
 		mc->mc_flags &= ~C_EOF;