t0090: avoid passing empty string to printf %d

FreeBSD's printf(1) doesn't accept empty strings for numerical format
specifiers:

	$ printf "%d\n" "" >/dev/null; echo $?
	printf: : expected numeric value
	1

Initialize the AWK variable c to make sure the shell variable
subtree_count always contains a numerical value, in order to keep the
subsequently called printf happy.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>