Fix NULL pointer handling in conflict resolver (#81)

According to [the
documentation](https://docs.couchbase.com/mobile/3.2.4/couchbase-lite-c/C/html/group__documents.html#ga6f9eaf3d038f8359b9d9fd3badc51be7)
the second parameter (the remote document) can be NULL if the document
is deleted in central.

Wrapping a NULL pointer in a `Document` instance can lead to undefined
behavior or crashes, so the `ConflictHandler` function now uses an
Option for its second parameter.

Author: Nao-ris

src/document.rs

@@ -65,7 +65,8 @@ pub enum ConcurrencyControl {
 /// if the save would cause a conflict, i.e. if the document in the database has been updated
 /// (probably by a pull replicator, or by application code on another thread)
 /// since it was loaded into the CBLDocument being saved.
-type ConflictHandler = fn(&mut Document, &Document) -> bool;
+/// Return true to save the document, false to cancel the save.
+type ConflictHandler = fn(&mut Document, Option<&Document>) -> bool;
 #[unsafe(no_mangle)]
 unsafe extern "C" fn c_conflict_handler(
     context: *mut ::std::os::raw::c_void,
@@ -74,10 +75,15 @@ unsafe extern "C" fn c_conflict_handler(
 ) -> bool {
     unsafe {
         let callback: ConflictHandler = std::mem::transmute(context);
+        let remote_document = &Document::reference(conflicting_document as *mut CBLDocument);
 
         callback(
             &mut Document::reference(document_being_saved),
-            &Document::reference(conflicting_document as *mut CBLDocument),
+            if conflicting_document.is_null() {
+                None
+            } else {
+                Some(remote_document)
+            },
         )
     }
 }

tests/document_tests.rs

@@ -206,7 +206,7 @@ fn database_save_document_resolving() {
         document = default_collection(db)
             .save_document_resolving(&mut document, |document_a, document_b| {
                 let property_a = document_a.properties().get("foo").as_i64_or_0();
-                let property_b = document_b.properties().get("foo").as_i64_or_0();
+                let property_b = document_b.unwrap().properties().get("foo").as_i64_or_0();
                 document_a
                     .mutable_properties()
                     .at("foo")