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

apply.whitespace configuration expects a string value.

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

builtin-apply.c

@@ -2746,6 +2746,8 @@ static int apply_patch(int fd, const char *filename, int inaccurate_eof)
 static int git_apply_config(const char *var, const char *value)
 {
 	if (!strcmp(var, "apply.whitespace")) {
+		if (!value)
+			return config_error_nonbool(var);
 		apply_default_whitespace = xstrdup(value);
 		return 0;
 	}