Remove redundant null check

crl_file is a test argument and can't be null. Besides the code above
already assumes that it's non-null.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/suites/test_suite_x509parse.function

@@ -674,7 +674,7 @@ void x509_verify(char *crt_file, char *ca_file, char *crl_file,
 #if defined(MBEDTLS_X509_TRUSTED_CERTIFICATE_CALLBACK)
     /* CRLs aren't supported with CA callbacks, so skip the CA callback
      * version of the test if CRLs are in use. */
-    if (crl_file == NULL || strcmp(crl_file, "") == 0) {
+    if (strcmp(crl_file, "") == 0) {
         flags = 0;
 
         res = mbedtls_x509_crt_verify_with_ca_cb(&crt,