git-format-patch: do not crash with format.headers without value.
git-format-patch: do not crash with format.headers without value.

An incorrect config file can say:

	[format]
		headers

and crash the parsing.

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

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