Correct c-struct 'null' checks

In luajit, a null pointer is distinct from nil, and thus "truthy". That
is to say, `if not c_ffi_struct` will never evaluate to true, even if
the struct is null.

However, a null ffi struct does compare equal to nil, so the new
comparison corrects this issue.

This fixes numerous bugs in this library, including multiple cases of
uninitialized data being passed into functions leading to null pointer
dereferences, buffer overflows, and more.

lib/resty/evp.lua

@@ -137,7 +137,7 @@ function RSASigner.new(self, pem_private_key)
     ffi.gc(rsa, _C.RSA_free)
 
     local evp_pkey = _C.EVP_PKEY_new()
-    if not evp_pkey then
+    if evp_pkey == nil then
         return _err()
     end
     ffi.gc(evp_pkey, _C.EVP_PKEY_free)
@@ -158,13 +158,13 @@ function RSASigner.sign(self, message, digest_name)
     local len = ffi.new("size_t[1]", 1024)
 
     local ctx = _C.EVP_MD_CTX_create()
-    if not ctx then
+    if ctx == nil then
         return _err()
     end
     ffi.gc(ctx, _C.EVP_MD_CTX_destroy)
 
     local md = _C.EVP_get_digestbyname(digest_name)
-    if not md then
+    if md == nil then
         return _err()
     end
 
@@ -209,12 +209,12 @@ end
 -- @returns bool, error_string
 function RSAVerifier.verify(self, message, sig, digest_name)
     local md = _C.EVP_get_digestbyname(digest_name)
-    if not md then
+    if md == nil then
         return _err(false)
     end
 
     local ctx = _C.EVP_MD_CTX_create()
-    if not ctx then
+    if ctx == nil then
         return _err(false)
     end
     ffi.gc(ctx, _C.EVP_MD_CTX_destroy)
@@ -265,7 +265,7 @@ function Cert.new(self, payload)
         end
         x509 = _C.d2i_X509_bio(bio, nil)
     end
-    if not x509 then
+    if x509 == nil then
         return _err()
     end
     ffi.gc(x509, _C.X509_free)
@@ -299,7 +299,7 @@ end
 -- @returns fingerprint_string
 function Cert.get_fingerprint(self, digest_name)
     local md = _C.EVP_get_digestbyname(digest_name)
-    if not md then
+    if md == nil then
         return _err()
     end
     local buf = ffi.new("unsigned char[?]", 32)
@@ -317,7 +317,7 @@ end
 -- @returns An OpenSSL EVP PKEY object representing the public key
 function Cert.get_public_key(self)
     local evp_pkey = _C.X509_get_pubkey(self.x509)
-    if not evp_pkey then
+    if evp_pkey == nil then
         return _err()
     end
 
@@ -329,7 +329,7 @@ end
 -- @return bool, error_string
 function Cert.verify_trust(self, trusted_cert_file)
     local store = _C.X509_STORE_new()
-    if not store then
+    if store == nil then
         return _err(false)
     end
     ffi.gc(store, _C.X509_STORE_free)
@@ -338,7 +338,7 @@ function Cert.verify_trust(self, trusted_cert_file)
     end
 
     local ctx = _C.X509_STORE_CTX_new()
-    if not store then
+    if store == nil then
         return _err(false)
     end
     ffi.gc(ctx, _C.X509_STORE_CTX_free)
@@ -386,7 +386,7 @@ function PublicKey.new(self, payload)
         end
         pkey = _C.d2i_PUBKEY_bio(bio, nil)
     end
-    if not pkey then
+    if pkey == nil then
         return _err()
     end
     ffi.gc(pkey, _C.EVP_PKEY_free)