message: Don't crash if stepping back from last instruction
diff: @@ -1089,7 +1089,8 @@ bool ShaderViewer::stepBack()
  {
  const ShaderDebugState &oldstate = m_Trace->states[CurrentStep()];
  - LineColumnInfo oldLine = m_Trace->lineInfo[oldstate.nextInstruction];
  + LineColumnInfo oldLine =
  + m_Trace->lineInfo[qMax(m_Trace->lineInfo.size() - 1, (size_t)oldstate.nextInstruction)];
  while(CurrentStep() < m_Trace->states.count())
  {
  