Fix uninitialised field in ctrl_fileselect.

In commit 694d5184b72505a, I introduced the 'just_button' flag for
CTRL_FILESELECT controls, and in commit ddcd93ab1230b2b I added a use
of such a control with the flag set to true.

But I forgot to set it to false everywhere else, which caused an
assertion failure when selecting the Bell pane in Windows PuTTY. Oops.

dialog.c

@@ -402,6 +402,7 @@ dlgcontrol *ctrl_filesel(struct controlset *s, const char *label,
     c->fileselect.filter = filter;
     c->fileselect.for_writing = write;
     c->fileselect.title = dupstr(title);
+    c->fileselect.just_button = false;
     return c;
 }