xdiff: introduce XDL_CALLOC_ARRAY()

xdiff: introduce XDL_CALLOC_ARRAY()

Add a helper for allocating an array and initialize the elements to
zero. This is analogous to CALLOC_ARRAY() in the rest of the codebase
but it returns NULL on allocation failures rather than dying to
accommodate other users of libxdiff such as libgit2.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>