ceph_erasure_code_benchmark: fix parameter handling

Make sure k and m paramter are valid to prevent crash. Fix typo.

Fix for the following CID and other possible invalid combinations
of k/m parameter:

CID 1219466 (#1 of 1): Division or modulo by zero (DIVIDE_BY_ZERO)
 divide_by_zero: In expression rand() % (k + m), modulo by expression
 k + m which may be zero has undefined behavior.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>