builtin-log.c: guard config parser from value=NULL

format.subjectprefix configuration expects a string value.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-log.c

diff --git a/builtin-log.c b/builtin-log.c
index dcc9f81..9458428 100644
--- a/builtin-log.c
+++ b/builtin-log.c
@@ -219,7 +219,7 @@
 {
 	if (!strcmp(var, "format.subjectprefix")) {
 		if (!value)
-			die("format.subjectprefix without value");
+			config_error_nonbool(var);
 		fmt_patch_subject_prefix = xstrdup(value);
 		return 0;
 	}