ITS#7722 fix page_merge for collapsing root

Other cursors being fixed up are not necessarily height 1.

libraries/liblmdb/mdb.c

@@ -7161,10 +7161,13 @@ 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_pg[0] = mc->mc_pg[0];
+						int i;
-						m3->mc_snum = 1;
+						m3->mc_snum--;
-						m3->mc_top = 0;
+						m3->mc_top--;
-						m3->mc_ki[0] = m3->mc_ki[1];
+						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];
+						}
 					}
 				}
 			}