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

crypto/asn1/x_bignum.c

@@ -82,7 +82,7 @@ static int bn_secure_new(ASN1_VALUE **pval, const ASN1_ITEM *it)
 
 static void bn_free(ASN1_VALUE **pval, const ASN1_ITEM *it)
 {
-    if (!*pval)
+    if (*pval == NULL)
         return;
     if (it->size & BN_SENSITIVE)
         BN_clear_free((BIGNUM *)*pval);
@@ -96,7 +96,7 @@ static int bn_i2c(ASN1_VALUE **pval, unsigned char *cont, int *putype,
 {
     BIGNUM *bn;
     int pad;
-    if (!*pval)
+    if (*pval == NULL)
         return -1;
     bn = (BIGNUM *)*pval;
     /* If MSB set in an octet we need a padding byte */
@@ -133,7 +133,7 @@ static int bn_secure_c2i(ASN1_VALUE **pval, const unsigned char *cont, int len,
     int ret;
     BIGNUM *bn;
 
-    if (!*pval && !bn_secure_new(pval, it))
+    if (*pval == NULL && !bn_secure_new(pval, it))
         return 0;
 
     ret = bn_c2i(pval, cont, len, utype, free_cont, it);