Fix nbtree high key "continuescan" row compare bug.

Commit 29b64d1d mishandled skipping over truncated high key attributes
during row comparisons.  The row comparison key matching loop would loop
forever when a truncated attribute was encountered for a row compare
subkey.  Fix by following the example of other code in the loop: advance
the current subkey, or break out of the loop when the last subkey is
reached.

Add test coverage for the relevant _bt_check_rowcompare() code path.
The new test case is somewhat tied to nbtree implementation details,
which isn't ideal, but seems unavoidable.

src/backend/access/nbtree/nbtutils.c

@@ -1543,6 +1543,9 @@ _bt_check_rowcompare(ScanKey skey, IndexTuple tuple, int tupnatts,
 			 */
 			Assert(ScanDirectionIsForward(dir));
 			cmpresult = 0;
+			if (subkey->sk_flags & SK_ROW_END)
+				break;
+			subkey++;
 			continue;
 		}