fix the problem:repeatedly stop cause app crash

modules/videoio/src/cap_ios_video_camera.mm

@@ -129,31 +129,33 @@ static CGFloat DegreesToRadians(CGFloat degrees) {return degrees * M_PI / 180;}
 
 - (void)stop;
 {
-    [super stop];
+    if (self.running == YES) {
+        [super stop];
 
-    [videoDataOutput release];
-    if (videoDataOutputQueue) {
-        dispatch_release(videoDataOutputQueue);
-    }
+        [videoDataOutput release];
+        if (videoDataOutputQueue) {
+            dispatch_release(videoDataOutputQueue);
+        }
 
-    if (self.recordVideo == YES) {
-        if (self.recordAssetWriter) {
-            if (self.recordAssetWriter.status == AVAssetWriterStatusWriting) {
-                [self.recordAssetWriter finishWriting];
-                NSLog(@"[Camera] recording stopped");
-            } else {
-                NSLog(@"[Camera] Recording Error: asset writer status is not writing");
+        if (self.recordVideo == YES) {
+            if (self.recordAssetWriter) {
+                if (self.recordAssetWriter.status == AVAssetWriterStatusWriting) {
+                    [self.recordAssetWriter finishWriting];
+                    NSLog(@"[Camera] recording stopped");
+                } else {
+                    NSLog(@"[Camera] Recording Error: asset writer status is not writing");
+                }
+                [recordAssetWriter release];
             }
-            [recordAssetWriter release];
-        }
 
-        [recordAssetWriterInput release];
-        [recordPixelBufferAdaptor release];
-    }
+            [recordAssetWriterInput release];
+            [recordPixelBufferAdaptor release];
+        }
 
-    if (self.customPreviewLayer) {
-        [self.customPreviewLayer removeFromSuperlayer];
-        self.customPreviewLayer = nil;
+        if (self.customPreviewLayer) {
+            [self.customPreviewLayer removeFromSuperlayer];
+            self.customPreviewLayer = nil;
+        }
     }
 }