Switch to safe calloc wrapper
Switch to safe calloc wrapper

The mbedTLS backend was still using plain calloc. I refactored the code
to use the new safe calloc wrapper.

fmorgner/umurmur,fmorgner/umurmur,snowblind/umurmur,snowblind/umurmur,troxor/umurmur,troxor/umurmur