vpaes-x86[_64].pl: handle zero length in vpaes_cbc_encrypt.

PR: 2775

crypto/aes/asm/vpaes-x86.pl

@@ -843,6 +843,8 @@ $k_dsbo=0x2c0;		# decryption sbox final output
 	&mov	($out,&wparam(1));		# out
 	&mov	($round,&wparam(2));		# len
 	&mov	($key,&wparam(3));		# key
+	&sub	($round,16);
+	&jc	(&label("cbc_abort"));
 	&lea	($base,&DWP(-56,"esp"));
 	&mov	($const,&wparam(4));		# ivp
 	&and	($base,-16);
@@ -853,7 +855,6 @@ $k_dsbo=0x2c0;		# decryption sbox final output
 	&mov	(&DWP(48,"esp"),$base);
 
 	&mov	(&DWP(0,"esp"),$out);		# save out
-	&sub	($round,16);
 	&mov	(&DWP(4,"esp"),$key)		# save key
 	&mov	(&DWP(8,"esp"),$const);		# save ivp
 	&mov	($out,$round);			# $out works as $len
@@ -896,6 +897,7 @@ $k_dsbo=0x2c0;		# decryption sbox final output
 	&mov	($base,&DWP(8,"esp"));		# restore ivp
 	&mov	("esp",&DWP(48,"esp"));
 	&movdqu	(&QWP(0,$base),"xmm1");		# write IV
+&set_label("cbc_abort");
 &function_end("${PREFIX}_cbc_encrypt");
 
 &asm_finish();

crypto/aes/asm/vpaes-x86_64.pl

@@ -869,6 +869,8 @@ ${PREFIX}_cbc_encrypt:
 ___
 ($len,$key)=($key,$len);
 $code.=<<___;
+	sub	\$16,$len
+	jc	.Lcbc_abort
 ___
 $code.=<<___ if ($win64);
 	lea	-0xb8(%rsp),%rsp
@@ -887,7 +889,6 @@ ___
 $code.=<<___;
 	movdqu	($ivp),%xmm6		# load IV
 	sub	$inp,$out
-	sub	\$16,$len
 	call	_vpaes_preheat
 	cmp	\$0,${enc}d
 	je	.Lcbc_dec_loop
@@ -932,6 +933,7 @@ $code.=<<___ if ($win64);
 .Lcbc_epilogue:
 ___
 $code.=<<___;
+.Lcbc_abort:
 	ret
 .size	${PREFIX}_cbc_encrypt,.-${PREFIX}_cbc_encrypt
 ___