Unix Pageant: fix a double-free when adding keys.

I had freed the comment string coming back from pageant_add_keyfile,
but not NULLed out the pointer, so that the cleanup code at the end of
the function would have freed it again.

unix/uxpgnt.c

@@ -331,6 +331,7 @@ static int unix_add_keyfile(const char *filename_str)
     while (1) {
         char *passphrase = askpass(err);
         sfree(err);
+        err = NULL;
         if (!passphrase)
             break;