Fix encoding bug in i2c_ASN1_INTEGER

(Imported from upstream's 3661bb4e7934668bd99ca777ea8b30eedfafa871.)

Fix bug where i2c_ASN1_INTEGER mishandles zero if it is marked as
negative.

Thanks to Huzaifa Sidhpurwala <huzaifas@redhat.com> and Hanno Böck
<hanno@hboeck.de> for reporting this issue.

BUG=590615

(cherry-picked from c4eec0c16b02c97a62a95b6a08656c3a9ddb6baa)

Bug: 28175332
Change-Id: I8959e8ae01510a5924862a3f353be23130eee554
Reviewed-on: https://boringssl-review.googlesource.com/7199

Reviewed-by: David Benjamin <davidben@google.com>

src/crypto/asn1/a_int.c

@@ -125,6 +125,8 @@ int i2c_ASN1_INTEGER(ASN1_INTEGER *a, unsigned char **pp)
 		{
 		ret=a->length;
 		i=a->data[0];
+		if (ret == 1 && i == 0)
+			neg=0;
 		if (!neg && (i > 127)) {
 			pad=1;
 			pb=0;
@@ -158,7 +160,7 @@ int i2c_ASN1_INTEGER(ASN1_INTEGER *a, unsigned char **pp)
 		p += a->length - 1;
 		i = a->length;
 		/* Copy zeros to destination as long as source is zero */
-		while(!*n) {
+		while(!*n && i > 1) {
 			*(p--) = 0;
 			n--;
 			i--;