pageant.c: turn a bare 'free' into sfree.

In normal builds this makes no difference, but in Windows builds with
the Minefield diagnostic system turned on, free()ing a Minefield-
allocated object causes a crash. Apparently I haven't tested Pageant
under Minefield for ages - only PuTTY, talking to an ordinary Pageant
I'd already started up.

pageant.c

@@ -936,7 +936,7 @@ void pageant_forget_passphrases(void)
 	char *pp = index234(passphrases, 0);
 	smemclr(pp, strlen(pp));
 	delpos234(passphrases, 0);
-	free(pp);
+	sfree(pp);
     }
 }