MtProtoKit: Cancel the backup address lookup when a context is cleaned up

MTContext's cleanup disposed every pending action except the backup
address discovery. A context freed while a lookup was in flight (the login
screen's network on macOS) hit the DEBUG leaked-disposable assert, and in
release the lookup kept running after the context was gone.

Author: macosswift

submodules/MtProtoKit/Sources/MTContext.m

@@ -397,6 +397,9 @@ - (void)cleanup
 
     NSDictionary *transportSchemeDisposableByDatacenterId = _transportSchemeDisposableByDatacenterId;
 
+    id<MTDisposable> backupAddressListDisposable = _backupAddressListDisposable;
+    _backupAddressListDisposable = nil;
+    
     [[MTContext contextQueue] dispatchOnQueue:^
     {
         for (NSNumber *nDatacenterId in discoverDatacenterAddressActions)
@@ -447,6 +450,8 @@ - (void)cleanup
             id<MTDisposable> disposable = transportSchemeDisposableByDatacenterId[nDatacenterId];
             [disposable dispose];
         }
+        
+        [backupAddressListDisposable dispose];
     }];
 }