convert.c: guard config parser from value=NULL

filter.*.smudge and filter.*.clean configuration variables expect a
string value.

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

convert.c

@@ -326,14 +326,14 @@ static int read_convert_config(const char *var, const char *value)
 
 	if (!strcmp("smudge", ep)) {
 		if (!value)
-			return error("%s: lacks value", var);
+			return config_error_nonbool(var);
 		drv->smudge = strdup(value);
 		return 0;
 	}
 
 	if (!strcmp("clean", ep)) {
 		if (!value)
-			return error("%s: lacks value", var);
+			return config_error_nonbool(var);
 		drv->clean = strdup(value);
 		return 0;
 	}