Fix potential NULL pointer dereference in lzc_ioctl()

Users are allowed to pass NULL to resultp, but we unconditionally assume
that they never do. When an external user does pass NULL to resultp, we
dereference a NULL pointer.

Clang's static analyzer complained about this.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes openzfs#14008

lib/libzfs_core/libzfs_core.c

@@ -233,7 +233,7 @@ lzc_ioctl(zfs_ioc_t ioc, const char *name,
 			break;
 		}
 	}
-	if (zc.zc_nvlist_dst_filled) {
+	if (zc.zc_nvlist_dst_filled && resultp != NULL) {
 		*resultp = fnvlist_unpack((void *)(uintptr_t)zc.zc_nvlist_dst,
 		    zc.zc_nvlist_dst_size);
 	}