[crypto/asn1/x_bignum.c] Explicit test against NULL

[crypto/asn1/x_bignum.c] Explicit test against NULL

As a fixup to https://github.com/openssl/openssl/pull/9779 to better
conform to the project code style guidelines, this commit amends the
original changeset to explicitly test against NULL, i.e. writing

```
if (p != NULL)
```

rather than

```
if (!p)
```

Reviewed-by: Nicola Tuveri <nic.tuv@gmail.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/9881)