testutil/load.c: Add checks for file(name) == NULL

Reviewed-by: NTomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15120)

test/testutil/load.c

@@ -20,7 +20,7 @@ X509 *load_cert_pem(const char *file, OSSL_LIB_CTX *libctx)
     X509 *cert = NULL;
     BIO *bio = NULL;
 
-    if (!TEST_ptr(bio = BIO_new(BIO_s_file())))
+    if (!TEST_ptr(file) || !TEST_ptr(bio = BIO_new(BIO_s_file())))
         return NULL;
     if (TEST_int_gt(BIO_read_filename(bio, file), 0)
             && TEST_ptr(cert = X509_new_ex(libctx, NULL)))
@@ -30,17 +30,14 @@ X509 *load_cert_pem(const char *file, OSSL_LIB_CTX *libctx)
     return cert;
 }
 
-STACK_OF(X509) *load_certs_pem(const char *filename)
+STACK_OF(X509) *load_certs_pem(const char *file)
 {
     STACK_OF(X509) *certs;
     BIO *bio;
     X509 *x;
 
-    bio = BIO_new_file(filename, "r");
-
-    if (bio == NULL) {
+    if (!TEST_ptr(file) || (bio = BIO_new_file(file, "r")) == NULL)
         return NULL;
-    }
 
     certs = sk_X509_new_null();
     if (certs == NULL) {
@@ -74,7 +71,7 @@ EVP_PKEY *load_pkey_pem(const char *file, OSSL_LIB_CTX *libctx)
     EVP_PKEY *key = NULL;
     BIO *bio = NULL;
 
-    if (!TEST_ptr(bio = BIO_new(BIO_s_file())))
+    if (!TEST_ptr(file) || !TEST_ptr(bio = BIO_new(BIO_s_file())))
         return NULL;
     if (TEST_int_gt(BIO_read_filename(bio, file), 0))
         (void)TEST_ptr(key = PEM_read_bio_PrivateKey_ex(bio, NULL, NULL, NULL,