fixed comparison to stop integer wrap around -> no more crash in cvFindCornerSubPix

cvFindCornerSubpix is able to generate corner position updates far away from the image (if the determinant of the corner patch is too small). This leads to large positive numbers which lead to integer wrap arounds when added to src_size. This results in access errors in samplers.cpp (i.e. invalid memory accesses and thus crashes)
gcc 4.7.1, 64bit Alces HPC Linux