Viewer crash in llassert(mCurrentDiscardLevel >= 0) when changing draw distance

Repro:

• Launch viewer and log in
• Move to your avatar to a place on the edge of the region where you get a good view of the nearby region
• Open the Pref panel, go to the Graphic tab, click Advanced, increase the Draw Distance (from 64 to 128 for instance)
• Click OK
  -> the view crash while redrawing the nearby stuff that are now coming into view
  Expected: no crash...

Reason: the code reaches llassert(mCurrentDiscardLevel >= 0) and crashes, the invalid texture should be tested upstream and handled adequately I think.

Issue: When changing the draw distance, LLViewerWindow::restartDisplay() is called which, eventually, calls restoreGL() which iterates on all textures. Here's the code:

void LLImageGL::restoreGL()
{
for (std::set<LLImageGL*>::iterator iter = sImageList.begin();
iter != sImageList.end(); iter++)
{
LLImageGL* glimage = *iter;
if(glimage->getTexName())

{ llerrs << "tex name is not 0." << llendl ; }

if (glimage->mSaveData.notNull())
{
if (glimage->getComponents() && glimage->mSaveData->getComponents())

{ glimage->createGLTexture(glimage->mCurrentDiscardLevel, glimage->mSaveData, 0, TRUE, glimage->getCategory()); stop_glerror(); }

glimage->mSaveData = NULL; // deletes data
}
}
}

The problem is that some of the textures are not valid and have an mCurrentDiscardLevel == -1. When createGLTexture() is called on the first of those textures, this leads to the assert and, as expected, crashes the app.