Fix bounds for empty text nodes

_lineCount can be 0, so we get underflow if the alignment is one of
the *_BOTTOM_BASE_LINE.

src/osgText/TextBase.cpp

@@ -484,6 +484,14 @@ void TextBase::computePositions()
 
 void TextBase::computePositionsImplementation()
 {
+    _normal = osg::Vec3(0.0f,0.0f,1.0f);
+
+    if (_text.empty())
+    {
+        _offset = Vec3();
+        return;
+    }
+
     switch(_alignment)
     {
         case LEFT_TOP:      _offset.set(_textBB.xMin(),_textBB.yMax(),_textBB.zMin()); break;
@@ -506,8 +514,6 @@ void TextBase::computePositionsImplementation()
         case CENTER_BOTTOM_BASE_LINE:  _offset.set((_textBB.xMax()+_textBB.xMin())*0.5f,-_characterHeight*(1.0 + _lineSpacing)*(_lineCount-1),0.0f); break;
         case RIGHT_BOTTOM_BASE_LINE:  _offset.set(_textBB.xMax(),-_characterHeight*(1.0 + _lineSpacing)*(_lineCount-1),0.0f); break;
     }
-
-    _normal = osg::Vec3(0.0f,0.0f,1.0f);
 }
 
 bool TextBase::computeMatrix(osg::Matrix& matrix, osg::State* state) const