potential null dereference when calling openssl API


**Describe the bug**
There are some potential NULL dereference bugs.
In nmap/nping/Crypto.cc:134 and 174, calling EVP_CIPHER_CTX_set_padding without checking the parameter 1 might cause null dereference

**Expected behavior**
It's better to check the pointer before calling EVP_CIPHER_CTX_set_padding.

