rbf_font: sanitize loaded bitmap font before use (CONFIG_NO_BFNT)

[kitor]

In 05a99a7c5f4c0723568382f629bb8b44a89dd7bb I blindly believe font file to be valid if just header magic matches.

bfnt_load_from_card should be updated to:

• verify if loaded size > header size
• parse header
• count if maximum offset fits in loaded file