Fix allocation errors, detected using ASAN

Fix allocation errors, detected using ASAN

The test for VDEV_TYPE_INDIRECT is done after a memory allocation, and
could return from function without freeing it.  Since we don't need that
allocation yet, just postpone it.

Add a missing free() when buffer is no longer needed.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: João Carlos Mendes Luís <jonny@jonny.eng.br>
Closes #10193