G_DoPlayDemo: Make sure buffer is NULL so you can watch replays immediately after recording them.

This is yet another g_demo.c-specific hack, and has been marked as such.

src/g_demo.c

@@ -3060,6 +3060,10 @@ void G_DoPlayDemo(const char *defdemoname)
 	}
 	else
 	{
+		// FIXME: this file doesn't manage its memory and actually free this when it's done using it
+		Z_Free(demobuf.buffer);
+		demobuf.buffer = NULL;
+
 		n = defdemoname+strlen(defdemoname);
 		while (*n != '/' && *n != '\\' && n != defdemoname)
 			n--;