Sev/menu image fix

I noticed a few error messages during start-up (see below). These are caused by a XonoticScreenshotImage object, which is initialized with an empty (string_null) image source. On a resizeNotify call, the null string is handed down a long sequence of functions and ultimately causes the errors.

This branch adds a check to the Image base class to prevent this.

FS_OpenVirtualFile("dds/gfx/menu/luminos/.dds", true): nasty filename rejected FS_OpenVirtualFile("gfx/menu/luminos/.tga", true): nasty filename rejected FS_OpenVirtualFile("gfx/menu/luminos/.png", true): nasty filename rejected FS_OpenVirtualFile("gfx/menu/luminos/.jpg", true): nasty filename rejected FS_OpenVirtualFile("gfx/menu/luminos/.pcx", true): nasty filename rejected FS_OpenVirtualFile("menu/luminos/.tga", true): nasty filename rejected FS_OpenVirtualFile("menu/luminos/.png", true): nasty filename rejected FS_OpenVirtualFile("menu/luminos/.jpg", true): nasty filename rejected FS_OpenVirtualFile("menu/luminos/.pcx", true): nasty filename rejected FS_OpenVirtualFile("gfx/menu/luminos/.lmp", false): nasty filename rejected