openssl: fix gcc8 warning

- Use memcpy instead of strncpy to copy a string without termination,
  since gcc8 warns about using strncpy to copy as many bytes from a
  string as its length.

Suggested-by: Viktor Szakats

Closes https://github.com/curl/curl/issues/2980

lib/vtls/openssl.c

@@ -253,7 +253,7 @@ static void ossl_keylog_callback(const SSL *ssl, const char *line)
       if(!buf)
         return;
     }
-    strncpy(buf, line, linelen);
+    memcpy(buf, line, linelen);
     buf[linelen] = '\n';
     buf[linelen + 1] = '\0';