Fix that skipping fails if target is 2nd frame

With bSeekForwardOnly the current file position should be an inclusive boundary,
but because the binary search stops if the search region (high-low) becomes
header_size, the current frame wasn't found. Because trjconv always reads the
first frame before calling seek, it failed if the target was the 2nd frame.

Fixes #1154

Change-Id: Id4151f0106abdb68d6067f02fe2927a4ec6d77a0