patch 8.0.0988: warning from Covscan about using NULL pointer

Problem:    Warning from Covscan about using NULL pointer.
Solution:   Add extra check for NULL. (zdohnal)
diff --git a/src/fileio.c b/src/fileio.c
index f54fb84..87a46b1 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -1370,7 +1370,8 @@
 		 * Decrypt the read bytes.  This is done before checking for
 		 * EOF because the crypt layer may be buffering.
 		 */
-		if (cryptkey != NULL && size > 0)
+		if (cryptkey != NULL && curbuf->b_cryptstate != NULL
+								   && size > 0)
 		{
 		    if (crypt_works_inplace(curbuf->b_cryptstate))
 		    {