Add more HRESULT checks

On multiple places, there is an interaction with endpointVolume and the function is expected to return something, 
but the return value of endpointVolume is not checked. Therefore, undefined values may be returned.
This commit adds return value checks with default values as return values for failing endpointVolume interactions.
This should fix null pointer dereference issues.

cppsrc/win/win-sound-mixer.cpp

@@ -381,14 +381,22 @@ void Device::SetMute(bool volume)
 float Device::GetVolume()
 {
     float volume;
-    endpointVolume->GetMasterVolumeLevelScalar(&volume);
+    HRESULT res = endpointVolume->GetMasterVolumeLevelScalar(&volume);
+    if (res != S_OK) {
+        return 0.F;
+    }
     return volume;
 }
 
 bool Device::GetMute()
 {
     BOOL mute;
-    endpointVolume->GetMute(&mute);
+    HRESULT res = endpointVolume->GetMute(&mute);
+    if (res != S_OK) {
+        // if we can't retrieve the GetMute state, there is probably something wrong with the audio endpoint 
+        // and the user can't hear it anyway, so just return true for the mute state.
+        return true;
+    }
     return (bool)mute;
 }
 
@@ -423,9 +431,17 @@ VolumeBalance Device::GetVolumeBalance()
         return result;
     }
     result.stereo = true;
-    endpointVolume->GetChannelVolumeLevelScalar(RIGHT, &result.right);
-    endpointVolume->GetChannelVolumeLevelScalar(LEFT, &result.left);
+    HRESULT res1 = endpointVolume->GetChannelVolumeLevelScalar(RIGHT, &result.right);
+    if (res1 != S_OK) {
+        result.right = 0.F; // revert right volume
+        return result;
+    }
 
+    HRESULT res2 = endpointVolume->GetChannelVolumeLevelScalar(LEFT, &result.left);
+    if (res2 != S_OK) {
+        result.left = 0.F; // revert left volume
+        return result;
+    }
     return result;
 }