[memutil] Allocate the right number of bytes in StagedMem::GetFlat()
[memutil] Allocate the right number of bytes in StagedMem::GetFlat()

This was an off-by-one error (forgetting that max_addr_ was inclusive).

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan