SampleBuffer: more sanity checks in getSampleFragment()

Despite of the previously introduced sanitizing it still sometimes happens
that an end frame variable is not greater than the corresponding start
frame variable. Make sure we don't crash by adding more sanity checks in
getSampleFragment().

Closes #629.

src/core/SampleBuffer.cpp

@@ -752,14 +752,14 @@ sampleFrame * SampleBuffer::getSampleFragment( f_cnt_t _start,
 {
 	if( _looped )
 	{
-		if( _start + _frames <= m_loopEndFrame )
+		if( _start + _frames <= m_loopEndFrame || m_loopStartFrame >= m_loopEndFrame )
 		{
 			return m_data + _start;
 		}
 	}
 	else
 	{
-		if( _start + _frames <= m_endFrame )
+		if( _start + _frames <= m_endFrame || m_startFrame >= m_endFrame )
 		{
 			return m_data + _start;
 		}