table_blocks computed before entries/entsz validation (fragile ordering)

Summary

entries*entsz multiply at :134 computed BEFORE bounds check at :136-139. Currently safe: post-check forces entries<=128 entsz<=MAXBSIZE/entries -> product <=65536. But future relaxation of MAX_GPT_ENTRIES -> 32-bit overflow -> undersized buffer -> heap OOB. Fragile.