bn/asm/rsaz-avx2.pl: fix occasional failures.

(Imported from upstream's 1067663d852435b1adff32ec01e9b8e54d2b5896)

Change-Id: I39e2a24176306f4170449145d3dee2c2edbf6dfe
diff --git a/crypto/bn/asm/rsaz-avx2.pl b/crypto/bn/asm/rsaz-avx2.pl
index bd4a6a8..85e1ce9 100644
--- a/crypto/bn/asm/rsaz-avx2.pl
+++ b/crypto/bn/asm/rsaz-avx2.pl
@@ -986,6 +986,7 @@
 
 	vmovdqu	.Land_mask(%rip), $AND_MASK
 	mov	\$9, $i
+	vmovdqu	$ACC9, 32*9-128($rp)		# $ACC9 is zero after vzeroall
 	jmp	.Loop_mul_1024
 
 .align	32