t0090: avoid passing empty string to printf %d
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 <2da278a99b557fc93ddbed7517a413ad793e2e6a@web.de>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git