libdtrace: use calloc(3) instead of malloc(3) when it makes sense.

libdtrace: use calloc(3) instead of malloc(3) when it makes sense.

calloc(3) is faster and occasionally safer than malloc(3) + bzero(3).

In one case, pointed out by Mark[1], this also cleans up a calculation.

Reviewed by:	markj [1]
MFC after:	1 week