Fix 0 byte memory leak in zfs receive

Currently, when a DRR_OBJECT record is read into memory in
receive_read_record(), memory is allocated for the bonus buffer.
However, if the object doesn't have a bonus buffer the code will
still "allocate" the zero bytes, but the memory will not be passed
to the processing thread for cleanup later. This causes the spl
kmem tracking code to report a leak. This patch simply changes the
code so that it only allocates this memory if it has a non-zero
length.

Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8266