Fix the crash when setting the fps of the virtual camera. (#62)

Author: cloudwebrtc

sdk/objc/components/capturer/RTCCameraVideoCapturer.m

@@ -495,7 +495,9 @@ - (void)updateDeviceCaptureFormat:(AVCaptureDeviceFormat *)format fps:(NSInteger
            @"updateDeviceCaptureFormat must be called on the capture queue.");
   @try {
     _currentDevice.activeFormat = format;
-    _currentDevice.activeVideoMinFrameDuration = CMTimeMake(1, fps);
+    if(![NSStringFromClass([_currentDevice class]) isEqualToString:@"AVCaptureDALDevice"]) {
+        _currentDevice.activeVideoMinFrameDuration = CMTimeMake(1, fps);
+    }
   } @catch (NSException *exception) {
     RTCLogError(@"Failed to set active format!\n User info:%@", exception.userInfo);
     return;