egg-secure-memory: Add a warning if #36 happens again

If we call sec_block_create() to allocate a block sized to hold `length`
bytes, and we succeed, then we should really be able to sec_alloc()
a cell of `length` bytes within that block. If we can't, then we chose
the size of the block incorrectly.

Signed-off-by: Simon McVittie <smcv@debian.org>