Fix buffer overflow in writeHashEntry()

MAXALIGN(datum_size) can be greater than BUFFER_INCREMENT_SIZE, hence
when allocating the buffer need to make sure buffer has enough space
for writing the next datum.

(cherry picked from commit bb99c441)

src/backend/executor/execHHashagg.c

@@ -1626,11 +1626,13 @@ writeHashEntry(AggState *aggstate, BatchFileInfo *file_info,
 
 		if ((aggDataOffset + MAXALIGN(datum_size)) >= aggDataBufferSize)
 		{
-			aggDataBufferSize += BUFFER_INCREMENT_SIZE;
+			aggDataBufferSize += MAXALIGN(datum_size) >= BUFFER_INCREMENT_SIZE ?
+				MAXALIGN(datum_size) + BUFFER_INCREMENT_SIZE : BUFFER_INCREMENT_SIZE;
 			MemoryContext oldAggContext = MemoryContextSwitchTo(TopMemoryContext);
 			aggDataBuffer = repalloc(aggDataBuffer, aggDataBufferSize);
 			MemoryContextSwitchTo(oldAggContext);
 		}
+		Assert((aggDataOffset + MAXALIGN(datum_size)) <= aggDataBufferSize);
 		memcpy((aggDataBuffer + aggDataOffset), datum_value, datum_size);
 
 		aggDataOffset += MAXALIGN(datum_size);