Fix coverity defects: CID 147587

CID 147587: Out-of-bounds read

Future changes may cause an array overrun of 4096 bytes at byte
offset 4096 by dereferencing pointer dstp. Adding this additional
check ensures correctness.

Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: GeLiXin <ge.lixin@zte.com.cn>
Closes #5297