Fix crash when canceling NSURLSession task

- The crash happened when canceling the task after read/write socket was closed.
- In -checkError:, also ignore the ENOTCONN error if there was a request socket close.
- In -readHTTPResponse, also call checkError: the same way as in -receive and -writeAndFree:. This is not directly related to the crash but make the error check going the same path.

#2085

Objective-C/Internal/Replicator/CBLWebSocket.mm

@@ -235,10 +235,9 @@ - (void) readHTTPResponse {
              completionHandler: ^(NSData* data, BOOL atEOF, NSError* error)
     {
         CBLLogVerbose(WebSocket, @"Received %zu bytes of HTTP response", (size_t)data.length);
-        if (error) {
-            [self didCloseWithError: error];
+        if ([self checkError: error])
             return;
-        }
+
         if (!CFHTTPMessageAppendBytes(httpResponse, (const UInt8*)data.bytes, data.length)) {
             // Error reading response!
             [self didCloseWithCode: kWebSocketCloseProtocolError
@@ -491,7 +490,7 @@ - (void)URLSession:(NSURLSession *)session
 
 
 - (bool) checkError: (NSError*)error {
-    if (!error)
+    if (!error || [self ignoreError: error])
         return false;
 
     _cancelError = error;
@@ -522,11 +521,8 @@ - (void) didCloseWithError: (NSError*)error {
         return;
     _task = nil;
 
-    // We sometimes get bogus(?) ENOTCONN errors after closing the socket.
-    if (_requestedClose && [error my_hasDomain: NSPOSIXErrorDomain code: ENOTCONN]) {
-        CBLLog(WebSocket, @"CBLWebSocket ignoring %@", error.my_compactDescription);
+    if ([self ignoreError: error])
         error = nil;
-    }
 
     C4Error c4err;
     if (error) {
@@ -542,6 +538,15 @@ - (void) didCloseWithError: (NSError*)error {
 }
 
 
+- (BOOL) ignoreError: (NSError*)error {
+    // We sometimes get bogus(?) ENOTCONN errors after closing the socket.
+    if (_requestedClose && [error my_hasDomain: NSPOSIXErrorDomain code: ENOTCONN]) {
+        CBLLog(WebSocket, @"CBLWebSocket ignoring %@", error.my_compactDescription);
+        return YES;
+    }
+    return NO;
+}
+
 #pragma mark - UTILITIES: