Allocate default-sized offset buffer to avoid validate-then-write regression

(valueCount + 1) * OFFSET_WIDTH produces a 4-byte buffer for valueCount=0,
which is too small when validateFull() triggers getFieldBuffers() before
the vector is populated — subsequent setVector() writes at index 4+ fail.

Use max(required, INITIAL_VALUE_ALLOCATION * OFFSET_WIDTH) to match the
default allocateNew() size. Use allocator.buffer() directly instead of
allocateOffsetBuffer() to avoid mutating offsetAllocationSizeInBytes.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Change-Id: Iccb00d4775b196f7978b34cbc98234f50ede370a

Author: prashanthbdremio

vector/src/main/java/org/apache/arrow/vector/complex/LargeListVector.java

@@ -280,7 +280,11 @@ public List<ArrowBuf> getFieldBuffers() {
     if (offsetBuffer.capacity() == 0 && offsetBuffer.writerIndex() > 0) {
       long writerIdx = offsetBuffer.writerIndex();
       offsetBuffer.getReferenceManager().release();
-      offsetBuffer = allocateOffsetBuffer((long) (valueCount + 1) * OFFSET_WIDTH);
+      long allocSize =
+          Math.max((long) (valueCount + 1) * OFFSET_WIDTH, INITIAL_VALUE_ALLOCATION * OFFSET_WIDTH);
+      offsetBuffer = allocator.buffer(allocSize);
+      offsetBuffer.setZero(0, offsetBuffer.capacity());
+      offsetBuffer.readerIndex(0);
       offsetBuffer.writerIndex(writerIdx);
     }
     result.add(offsetBuffer);

vector/src/main/java/org/apache/arrow/vector/complex/ListVector.java

@@ -238,7 +238,11 @@ public List<ArrowBuf> getFieldBuffers() {
     if (offsetBuffer.capacity() == 0 && offsetBuffer.writerIndex() > 0) {
       long writerIdx = offsetBuffer.writerIndex();
       offsetBuffer.getReferenceManager().release();
-      offsetBuffer = allocateOffsetBuffer((long) (valueCount + 1) * OFFSET_WIDTH);
+      long allocSize =
+          Math.max((long) (valueCount + 1) * OFFSET_WIDTH, INITIAL_VALUE_ALLOCATION * OFFSET_WIDTH);
+      offsetBuffer = allocator.buffer(allocSize);
+      offsetBuffer.setZero(0, offsetBuffer.capacity());
+      offsetBuffer.readerIndex(0);
       offsetBuffer.writerIndex(writerIdx);
     }
     result.add(offsetBuffer);