vt: prevent kernel-infoleak in con_font_get()

In the Linux kernel, the following vulnerability has been resolved: vt: prevent kernel-infoleak in con_font_get() font.data may not initialize all memory spaces depending on the implementation of vc->vc_sw->con_font_get. This may cause info-leak, so to prevent this, it is safest to modify it to initialize the allocated memory space to 0, and it generally does not affect the overall performance of the system.