Make imagelib not blow away images in the image cache on 404. Bug 349200,

r=pavlov, sr=darin

modules/libpr0n/src/imgRequest.cpp

@@ -291,7 +291,9 @@ void imgRequest::Cancel(nsresult aStatus)
   if (!(mImageStatus & imgIRequest::STATUS_LOAD_PARTIAL))
     mImageStatus |= imgIRequest::STATUS_ERROR;
 
+  if (aStatus != NS_IMAGELIB_ERROR_NO_DECODER) {
     RemoveFromCache();
+  }
 
   if (mRequest && mLoading)
     mRequest->Cancel(aStatus);