girparser: Fix criticals on calculate_common_prefix

diff --git a/vala/valagirparser.vala b/vala/valagirparser.vala
index 1908a0527dc3e59299957d0e9c69a9b706653711..615284151e62c8902c311d5178eda7cf4de684be 100644 (file)
--- a/vala/valagirparser.vala
+++ b/vala/valagirparser.vala
@@ -1932,7 +1932,7 @@ public class Vala.GirParser : CodeVisitor {
                end_element ("alias");
        }
 
-       private void calculate_common_prefix (ref string common_prefix, string cname) {
+       private void calculate_common_prefix (ref string? common_prefix, string cname) {
                if (common_prefix == null) {
                        common_prefix = cname;
                        while (common_prefix.length > 0 && !common_prefix.has_suffix ("_")) {