Force nul termination of the kstr holding the CRAM index so that
sscanf doesn't attempt to check beyond the bounds of the string
memory.

On correctly formatted indices it has no need to do this (but
apparently does), but the fix is necessary anyway in order to be
robust on invalid indices.
Ensure hand-made kstring is NUL-terminated

The kputsn()s used to load the file into memory do NUL-terminate (like
most other kstring functions), so we only need to ensure this by hand
for the hand-made kstring made from zlib_mem_inflate().