cgen: fix empty shared struct init (fix #16234) (#26354)

Author: kbkpbot

vlib/v/gen/c/struct.v

@@ -473,6 +473,12 @@ fn (mut g Gen) zero_struct_field(field ast.StructField) bool {
 	field_name := if sym.language == .v { c_name(field.name) } else { field.name }
 	if sym.info is ast.Struct {
 		if sym.info.fields.len == 0 {
+			// For empty struct fields, check if it's a shared field
+			if field.typ.has_flag(.shared_f) {
+				g.write('.${field_name} = ')
+				g.init_shared_field(field)
+				return true
+			}
 			return false
 		} else if !field.has_default_expr {
 			mut has_option_field := false

vlib/v/tests/concurrency/shared_elem_test.v

@@ -168,3 +168,21 @@ fn test_array_of_shared() {
 	assert e == 3
 	assert f == -17
 }
+
+// Test for issue #16234: empty shared struct initialized as null causing segmentation fault
+struct EmptyStruct {}
+
+struct OwnerWithEmptyShared {
+	empty EmptyStruct
+	lck   shared EmptyStruct
+}
+
+fn test_shared_empty_struct() {
+	mut i := OwnerWithEmptyShared{}
+	dump(i)
+	// Test that we can lock the empty shared struct without segfault
+	lock i.lck {
+		// Empty struct, nothing to do
+	}
+	assert true
+}