[MINOR][SQL][FOLLOW-UP] Add assertion to FixedLengthRowBasedKeyValueBatch

### What changes were proposed in this pull request?
Adds an assert to `FixedLengthRowBasedKeyValueBatch#appendRow` method to check the incoming vlen and klen by comparing them with the lengths stored as member variables as followup to apache#30788

### Why are the changes needed?
Add assert statement to catch similar bugs in future.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
Ran some tests locally, though not easy to test.

Closes apache#31447 from yliou/SPARK-33726-Assert.

Authored-by: yliou <yliou@berkeley.edu>
Signed-off-by: Sean Owen <srowen@gmail.com>

Author: yliou

sql/catalyst/src/main/java/org/apache/spark/sql/catalyst/expressions/FixedLengthRowBasedKeyValueBatch.java

@@ -46,6 +46,8 @@ private long getKeyOffsetForFixedLengthRecords(int rowId) {
   public UnsafeRow appendRow(Object kbase, long koff, int klen,
                              Object vbase, long voff, int vlen) {
     // if run out of max supported rows or page size, return null
+    assert(vlen == this.vlen);
+    assert(klen == this.klen);
     if (numRows >= capacity || page == null || page.size() - pageCursor < recordLength) {
       return null;
     }