coverity#441137 Dereference after null check

Change-Id: I877abe904885f28560cf43ee70001ac97fadbe04

forms/source/component/imgprod.cxx

@@ -333,6 +333,9 @@ void ImageProducer::startProduction() throw(::com::sun::star::uno::RuntimeExcept
 
 sal_Bool ImageProducer::ImplImportGraphic( Graphic& rGraphic )
 {
+    if (!mpStm)
+        return false;
+
     if( ERRCODE_IO_PENDING == mpStm->GetError() )
         mpStm->ResetError();