Fix crash in zio_done error reporting

Fix NULL pointer dereference when reporting
checksum error for gang block in zio_done.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Zuchowski <pzuchowski@datto.com>
Closes #11872
Closes #11896

module/zfs/zio.c

@@ -25,6 +25,7 @@
  * Copyright (c) 2017, Intel Corporation.
  * Copyright (c) 2019, Klara Inc.
  * Copyright (c) 2019, Allan Jude
+ * Copyright (c) 2021, Datto, Inc.
  */
 
 #include <sys/sysmacros.h>
@@ -4499,7 +4500,7 @@ zio_done(zio_t *zio)
 			uint64_t asize = P2ROUNDUP(psize, align);
 			abd_t *adata = zio->io_abd;
 
-			if (asize != psize) {
+			if (adata != NULL && asize != psize) {
 				adata = abd_alloc(asize, B_TRUE);
 				abd_copy(adata, zio->io_abd, psize);
 				abd_zero_off(adata, psize, asize - psize);
@@ -4510,7 +4511,7 @@ zio_done(zio_t *zio)
 			zcr->zcr_finish(zcr, adata);
 			zfs_ereport_free_checksum(zcr);
 
-			if (asize != psize)
+			if (adata != NULL && asize != psize)
 				abd_free(adata);
 		}
 	}