convert trivial cases to ALLOC_ARRAY

Each of these cases can be converted to use ALLOC_ARRAY or
REALLOC_ARRAY, which has two advantages:

  1. It automatically checks the array-size multiplication
     for overflow.

  2. It always uses sizeof(*array) for the element-size,
     so that it can never go out of sync with the declared
     type of the array.

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

alias.c

@@ -23,7 +23,7 @@ int split_cmdline(char *cmdline, const char ***argv)
 	int src, dst, count = 0, size = 16;
 	char quoted = 0;
 
-	*argv = xmalloc(sizeof(**argv) * size);
+	ALLOC_ARRAY(*argv, size);
 
 	/* split alias_string */
 	(*argv)[count++] = cmdline;