use correct format in conversion

webcam.cpp

@@ -146,10 +146,14 @@ const Image& Webcam::frame(int timeout)
                                      yres,
                                      stride);
             }
-            else if(format == V4L2_PIX_FMT_JPEG)
+            else if(format == V4L2_PIX_FMT_MJPEG)
             {
                 rgb_frame = decompressJpegImage((unsigned char *) buffers[idx].data, buffers[idx].size);
             }
+            else
+            {
+                assert(false);
+            }
             return rgb_frame;
         }
         /* EAGAIN - continue select loop. */