qcap: Fix a mem leak on an error path. Found by Smatch.

dlls/qcap/vfwcapture.c

@@ -767,6 +767,8 @@ VfwPin_Construct( IBaseFilter * pBaseFilter, LPCRITICAL_SECTION pCritSec,
         *ppPin = (IPin *)(&pPinImpl->pin.pin.lpVtbl);
         return S_OK;
     }
+
+    CoTaskMemFree(pPinImpl);
     return E_FAIL;
 }