add debug log for dqt parsing

src/MJPEGVideoSource.cpp

@@ -51,11 +51,12 @@ void MJPEGVideoSource::afterGettingFrame(unsigned frameSize,unsigned numTruncate
 		int qtable_length = length-2;
 		unsigned int qtable_position = i+4;
 		while (qtable_length > 0) {
+		    LOG(DEBUG) << "DQT qtable_length:" << qtable_length;
 		    unsigned int precision = (fTo[qtable_position]&0xf0)<<4;
 		    unsigned int quantIdx  = fTo[qtable_position]&0x0f;
 		    unsigned int quantSize =  64*(precision+1);
 		    if (quantSize*quantIdx+quantSize <= sizeof(m_qTable)) {
-			if ( (i+2+length) < frameSize) {
+			if ( (qtable_position+quantSize) < frameSize) {
 			    memcpy(m_qTable + quantSize*quantIdx, fTo + qtable_position + 1, quantSize);
 			    LOG(DEBUG) << "Quantization table idx:" << quantIdx << " precision:" << precision << " size:" << quantSize << " total size:" << m_qTableSize;
 			    if (quantSize*quantIdx+quantSize > m_qTableSize) {