vala: Add missing null-check in DataType.get_type_signature()

If an instance struct field can not be resolved to a valid
type-signature then bail.

This fixes criticals like:

    g_string_insert_len: assertion 'len == 0 || val != NULL' failed

1 files changed, 10 insertions, 0 deletions

diff --git a/tests/generics/gvariant-serialization.test b/tests/generics/gvariant-serialization.test
new file mode 100644
index 000000000..e9f9ad593
--- /dev/null
+++ b/tests/generics/gvariant-serialization.test
@@ -0,0 +1,10 @@
+Invalid Code
+
+struct Foo<G> {
+	public G g;
+}
+
+void main () {
+	var foo = new Foo<string>[] {};
+	Variant v = foo;
+}