Fix findContours crash for very large images (v2.4)

* Cast step to size_t in order to avoid integer overflow when processing very large images

* Change assert to CV_Assert

'const' is available in OpenCV 3.x only

fixed invalid output of cv::dft when using DFT_ROWS + DFT_COMPLEX_OUTPUT (http://code.opencv.org/issues/3428)

Replace asm by __asm__ according to
https://gcc.gnu.org/onlinedocs/gcc/Alternate-Keywords.html#Alternate-Keywords
as suggested by Arnout Vandecappelle:
http://lists.busybox.net/pipermail/buildroot/2016-September/171491.html

to fix build errors in ffmpeg with opencv2 support detected by
buildroot autobuilders:
http://autobuild.buildroot.net/results/c32/c32a21240a9933796ee850349a62ff3c2314f25c/build-end.logSigned-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

resolves #6944