Coverity Fix Null Check (aws#1965)

`X509_ATTRIBUTE_count` dereferences `a` without a NULL check, added it
in.

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under the terms of the Apache 2.0 license and the ISC license.

Author: smittals2

crypto/x509/x509_att.c

@@ -192,6 +192,9 @@ int X509_ATTRIBUTE_set1_data(X509_ATTRIBUTE *attr, int attrtype,
 }
 
 int X509_ATTRIBUTE_count(const X509_ATTRIBUTE *attr) {
+  if (attr == NULL) {
+    return 0;
+  }
   return (int)sk_ASN1_TYPE_num(attr->set);
 }
 

include/openssl/x509.h

@@ -2272,7 +2272,8 @@ OPENSSL_EXPORT int X509_ATTRIBUTE_set1_data(X509_ATTRIBUTE *attr, int attrtype,
 OPENSSL_EXPORT void *X509_ATTRIBUTE_get0_data(X509_ATTRIBUTE *attr, int idx,
                                               int attrtype, void *unused);
 
-// X509_ATTRIBUTE_count returns the number of values in |attr|.
+// X509_ATTRIBUTE_count returns the number of values in |attr| or 0 if |attr|
+// is NULL.
 OPENSSL_EXPORT int X509_ATTRIBUTE_count(const X509_ATTRIBUTE *attr);
 
 // X509_ATTRIBUTE_get0_object returns the type of |attr|.