nss: fix undefined behavior due to too large shift

When building with clang -fsanitize=undefined, ubsan says:

x509.c:1750:46: runtime error: shift exponent 64 is too large for 64-bit type 'PRUint64' (aka 'unsigned long')
    #0 0x444d45 in xmlSecNssASN1IntegerWrite src/nss/x509.c:1750:46
    lsh123#1 0x4443ec in xmlSecNssX509IssuerSerialNodeWrite src/nss/x509.c:1259:11
    lsh123#2 0x4403ba in xmlSecNssKeyDataX509XmlWrite src/nss/x509.c:769:19
    lsh123#3 0x45962a in xmlSecKeyInfoNodeWrite src/keyinfo.c:180:19
    lsh123#4 0x480149 in xmlSecDSigCtxProcessKeyInfoNode src/xmldsig.c:807:15
    lsh123#5 0x47c774 in xmlSecDSigCtxProcessSignatureNode src/xmldsig.c:506:11
    lsh123#6 0x47bfb2 in xmlSecDSigCtxSign src/xmldsig.c:289:11

And indeed shifting a 64bit value by 64 bits happens in practice there
as num->len is 9. At the same time (at least in case of the test) in all
3 cases the value that would be shifted is 0.

Avoid undefined behavior by simply not shifting if the value is 0
anyway.

Testcase: make check-crypto-nss XMLSEC_TEST_NAME="aleksey-xmldsig-01/x509data-sn-test"

src/nss/x509.c

@@ -1747,7 +1747,9 @@ xmlSecNssASN1IntegerWrite(SECItem *num) {
      * NSS bug http://bugzilla.mozilla.org/show_bug.cgi?id=212864 is fixed
      */
     for(ii = num->len; ii > 0; --ii, shift += 8) {
-        val |= ((PRUint64)num->data[ii - 1]) << shift;
+        if(num->data[ii - 1] != 0) {
+            val |= ((PRUint64)num->data[ii - 1]) << shift;
+        }
     }
 
     res = (xmlChar*)xmlMalloc(resLen + 1);