Read past end of argv array in zpool_do_import()

`zpool_do_import()` passes `argv[0]`, (optionally) `argv[1]`, and
`pool_specified` to `import_pools()`.  If `pool_specified==FALSE`, the
`argv[]` arguments are not used.  However, these values may be off the
end of the `argv[]` array, so loading them could dereference unmapped
memory.  This error is reported by the asan build:

```
=================================================================
==6003==ERROR: AddressSanitizer: heap-buffer-overflow
READ of size 8 at 0x6030000004a8 thread T0
    #0 0x562a078b50eb in zpool_do_import zpool_main.c:3796
    #1 0x562a078858c5 in main zpool_main.c:10709
    #2 0x7f5115231bf6 in __libc_start_main
    #3 0x562a07885eb9 in _start

0x6030000004a8 is located 0 bytes to the right of 24-byte region
allocated by thread T0 here:
    #0 0x7f5116ac6b40 in __interceptor_malloc
    #1 0x562a07885770 in main zpool_main.c:10699
    #2 0x7f5115231bf6 in __libc_start_main
```

This commit passes NULL for these arguments if they are off the end
of the `argv[]` array.

Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Allan Jude <allan@klarasystems.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #12339

cmd/zpool/zpool_main.c

@@ -3745,9 +3745,10 @@ zpool_do_import(int argc, char **argv)
 		return (1);
 	}
 
-	err = import_pools(pools, props, mntopts, flags, argv[0],
-	    argc == 1 ? NULL : argv[1], do_destroyed, pool_specified,
-	    do_all, &idata);
+	err = import_pools(pools, props, mntopts, flags,
+	    argc >= 1 ? argv[0] : NULL,
+	    argc >= 2 ? argv[1] : NULL,
+	    do_destroyed, pool_specified, do_all, &idata);
 
 	/*
 	 * If we're using the cachefile and we failed to import, then
@@ -3767,9 +3768,10 @@ zpool_do_import(int argc, char **argv)
 		nvlist_free(pools);
 		pools = zpool_search_import(g_zfs, &idata, &libzfs_config_ops);
 
-		err = import_pools(pools, props, mntopts, flags, argv[0],
-		    argc == 1 ? NULL : argv[1], do_destroyed, pool_specified,
-		    do_all, &idata);
+		err = import_pools(pools, props, mntopts, flags,
+		    argc >= 1 ? argv[0] : NULL,
+		    argc >= 2 ? argv[1] : NULL,
+		    do_destroyed, pool_specified, do_all, &idata);
 	}
 
 error: