compat/mingw: brown paper bag fix for 50a6c8ef

Commit 50a6c8ef (use st_add and st_mult for allocation size
computation, 2016-02-22) fixed up many xmalloc call-sites
including ones in compat/mingw.c.

But I screwed up one of them, which was half-converted to
ALLOC_ARRAY, using a very early prototype of the function.
And I never caught it because I don't build on Windows.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>