Fix some "'var' may be used uninitialized" warnings

The one in WPrefs.app/Appearance.c is indirect: the function might
be inlined and might return an uninitialized value, which causes gcc to
complain that the caller might use that uninitialized return value.

WPrefs.app/Appearance.c

@@ -704,6 +704,8 @@ static Pixmap renderMenu(_Panel * panel, WMPropList * texture, int width, int ih
 	case MSTYLE_FLAT:
 		pix = renderTexture(scr, texture, width, iheight * 4, NULL, RBEV_RAISED2);
 		break;
+	default:
+		pix = None;
 	}
 	XFreeGC(dpy, gc);