import: Use malloc() instead of calloc() when allocating memory buffer

import: Use malloc() instead of calloc() when allocating memory buffer

Since we're setting a sentinel at the end of the memory area anyway,
we really don't have to have the entire arena null'ed out.

Also make sure the sentinel is actually set. Previously it was dead
code, since it was after an unconditional crash() call. Now we set it
at the time we allocate the buffer.

On long-running systems where all memory is used for either programs
or caches, this provides a small static speedup.

Signed-off-by: Andreas Ericsson <ae@op5.se>