Test, don't just Assert, that mergejoin's inputs are in order.

There are two Asserts in nodeMergejoin.c that are reachable if
the input data is not in the expected order.  This seems way too
fragile.  Alexander Lakhin reported a case where the assertions
could be triggered with misconfigured foreign-table partitions,
and bitter experience with unstable operating system collation
definitions suggests another easy route to hitting them.  Neither
Assert is in a place where we can't afford one more test-and-branch,
so replace 'em with plain test-and-elog logic.

Per bug #17395.  While the reported symptom is relatively recent,
collation changes could happen anytime, so back-patch to all
supported branches.

Discussion: https://postgr.es/m/17395-8c326292078d1a57@postgresql.org

src/backend/executor/nodeMergejoin.c

@@ -893,11 +893,10 @@ ExecMergeJoin(PlanState *pstate)
 
 						if (compareResult == 0)
 							node->mj_JoinState = EXEC_MJ_JOINTUPLES;
-						else
-						{
-							Assert(compareResult < 0);
+						else if (compareResult < 0)
 							node->mj_JoinState = EXEC_MJ_NEXTOUTER;
-						}
+						else	/* compareResult > 0 should not happen */
+							elog(ERROR, "mergejoin input data is out of order");
 						break;
 					case MJEVAL_NONMATCHABLE:
 
@@ -1087,7 +1086,7 @@ ExecMergeJoin(PlanState *pstate)
 
 					node->mj_JoinState = EXEC_MJ_JOINTUPLES;
 				}
-				else
+				else if (compareResult > 0)
 				{
 					/* ----------------
 					 *	if the new outer tuple didn't match the marked inner
@@ -1106,7 +1105,6 @@ ExecMergeJoin(PlanState *pstate)
 					 *	no more inners, no more matches are possible.
 					 * ----------------
 					 */
-					Assert(compareResult > 0);
 					innerTupleSlot = node->mj_InnerTupleSlot;
 
 					/* reload comparison data for current inner */
@@ -1140,6 +1138,8 @@ ExecMergeJoin(PlanState *pstate)
 							return NULL;
 					}
 				}
+				else			/* compareResult < 0 should not happen */
+					elog(ERROR, "mergejoin input data is out of order");
 				break;
 
 				/*----------------------------------------------------------