Call of memcmp with null pointers in obj_cmp()

The function obj_cmp() (file crypto/objects/obj_dat.c) can in some
situations call memcmp() with a null pointer and a zero length.

This is invalid behaviour. When compiling openssl with undefined
behaviour sanitizer (add -fsanitize=undefined to compile flags) this
can be seen. One example that triggers this behaviour is the pkcs7
command (but there are others, e.g. I've seen it with the timestamp
function):
apps/openssl pkcs7 -in test/testp7.pem

What happens is that obj_cmp takes objects of the type ASN1_OBJECT and
passes their ->data pointer to memcmp. Zero-sized ASN1_OBJECT
structures can have a null pointer as data.

RT#3816

Signed-off-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(cherry picked from commit 2b8dc08b74fc3c6d4c2fc855cc23bac691d985be)

crypto/objects/obj_dat.c

@@ -400,6 +400,8 @@ static int obj_cmp(const ASN1_OBJECT *const *ap, const unsigned int *bp)
     j = (a->length - b->length);
     if (j)
         return (j);
+    if (a->length == 0)
+        return 0;
     return (memcmp(a->data, b->data, a->length));
 }