compat/mingw: brown paper bag fix for 50a6c8e

Commit 50a6c8e (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>

compat/mingw.c

@@ -1028,7 +1028,7 @@ static pid_t mingw_spawnve_fd(const char *cmd, const char **argv, char **deltaen
 			free(quoted);
 	}
 
-	wargs = xmalloc_array(st_add(st_mult(2, args.len), 1), sizeof(wchar_t));
+	ALLOC_ARRAY(wargs, st_add(st_mult(2, args.len), 1));
 	xutftowcs(wargs, args.buf, 2 * args.len + 1);
 	strbuf_release(&args);