[ruby/openssl] ssl: fix "warning: ‘ctx’ may be used uninitialized"

The code was introduced by https://github.com/ruby/openssl/commit/65530b887e54 ("ssl: enable generating keying
material from SSL sessions", 2022-08-03).

This is harmless, but we should avoid it.

https://github.com/ruby/openssl/commit/f5b82e814b

ext/openssl/ossl_ssl.c

@@ -2450,7 +2450,7 @@ ossl_ssl_export_keying_material(int argc, VALUE *argv, VALUE self)
     unsigned char *p;
     size_t len;
     int use_ctx = 0;
-    unsigned char *ctx;
+    unsigned char *ctx = NULL;
     size_t ctx_len = 0;
     int ret;