crypto: add missing null check

Add null check before using result of
ERR_reason_error_string. Coverity reported as an issue
and we seem to do a null check in other places we call
the function.

Signed-off-by: Michael Dawson <mdawson@devrus.com>

PR-URL: #40598
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>

src/crypto/crypto_context.cc

@@ -1037,6 +1037,8 @@ void SecureContext::LoadPKCS12(const FunctionCallbackInfo<Value>& args) {
     // TODO(@jasnell): Should this use ThrowCryptoError?
     unsigned long err = ERR_get_error();  // NOLINT(runtime/int)
     const char* str = ERR_reason_error_string(err);
+    str = str != nullptr ? str : "Unknown error";
+
     return env->ThrowError(str);
   }
 }